test: closeable: allocate variables accessed by continuations using do_with

Fixes failures in debug mode:
```
$ build/debug/tests/unit/closeable_test -l all -t deferred_close_test
WARNING: debug mode. Not for benchmarking or production
random-seed=3064133628
Running 1 test case...
Entering test module "../../tests/unit/closeable_test.cc"
../../tests/unit/closeable_test.cc(0): Entering test case "deferred_close_test"
../../src/testing/seastar_test.cc(43): info: check true has passed
==9449==WARNING: ASan doesn't fully support makecontext/swapcontext functions and may produce false positives in some cases!
terminate called after throwing an instance of 'seastar::broken_promise'
  what():  broken promise
==9449==WARNING: ASan is ignoring requested __asan_handle_no_return: stack top: 0x7fbf1f49f000; bottom 0x7fbf40971000; size: 0xffffffffdeb2e000 (-558702592)
False positive error reports may follow
For details see google/sanitizers#189
=================================================================
==9449==AddressSanitizer CHECK failed: ../../../../libsanitizer/asan/asan_thread.cpp:356 "((ptr[0] == kCurrentStackFrameMagic)) != (0)" (0x0, 0x0)
    #0 0x7fbf45f39d0b  (/lib64/libasan.so.6+0xb3d0b)
    #1 0x7fbf45f57d4e  (/lib64/libasan.so.6+0xd1d4e)
    #2 0x7fbf45f3e724  (/lib64/libasan.so.6+0xb8724)
    #3 0x7fbf45eb3e5b  (/lib64/libasan.so.6+0x2de5b)
    #4 0x7fbf45eb51e8  (/lib64/libasan.so.6+0x2f1e8)
    #5 0x7fbf45eb7694  (/lib64/libasan.so.6+0x31694)
    #6 0x7fbf45f39398  (/lib64/libasan.so.6+0xb3398)
    #7 0x7fbf45f3a00b in __asan_report_load8 (/lib64/libasan.so.6+0xb400b)
    #8 0xfe6d52 in bool __gnu_cxx::operator!=<dl_phdr_info*, std::vector<dl_phdr_info, std::allocator<dl_phdr_info> > >(__gnu_cxx::__normal_iterator<dl_phdr_info*, std::vector<dl_phdr_info, std::allocator<dl_phdr_info> > > const&, __gnu_cxx::__normal_iterator<dl_phdr_info*, std::vector<dl_phdr_info, std::allocator<dl_phdr_info> > > const&) /usr/include/c++/10/bits/stl_iterator.h:1116
    #9 0xfe615c in dl_iterate_phdr ../../src/core/exception_hacks.cc:121
    #10 0x7fbf44bd1810 in _Unwind_Find_FDE (/lib64/libgcc_s.so.1+0x13810)
    #11 0x7fbf44bcd897  (/lib64/libgcc_s.so.1+0xf897)
    #12 0x7fbf44bcea5f  (/lib64/libgcc_s.so.1+0x10a5f)
    #13 0x7fbf44bcefd8 in _Unwind_RaiseException (/lib64/libgcc_s.so.1+0x10fd8)
    #14 0xfe6281 in _Unwind_RaiseException ../../src/core/exception_hacks.cc:148
    scylladb#15 0x7fbf457364bb in __cxa_throw (/lib64/libstdc++.so.6+0xaa4bb)
    scylladb#16 0x7fbf45e10a21  (/lib64/libboost_unit_test_framework.so.1.73.0+0x1aa21)
    scylladb#17 0x7fbf45e20fe0 in boost::execution_monitor::execute(boost::function<int ()> const&) (/lib64/libboost_unit_test_framework.so.1.73.0+0x2afe0)
    scylladb#18 0x7fbf45e21094 in boost::execution_monitor::vexecute(boost::function<void ()> const&) (/lib64/libboost_unit_test_framework.so.1.73.0+0x2b094)
    scylladb#19 0x7fbf45e43921 in boost::unit_test::unit_test_monitor_t::execute_and_translate(boost::function<void ()> const&, unsigned long) (/lib64/libboost_unit_test_framework.so.1.73.0+0x4d921)
    scylladb#20 0x7fbf45e5eae1  (/lib64/libboost_unit_test_framework.so.1.73.0+0x68ae1)
    scylladb#21 0x7fbf45e5ed31  (/lib64/libboost_unit_test_framework.so.1.73.0+0x68d31)
    scylladb#22 0x7fbf45e2e547 in boost::unit_test::framework::run(unsigned long, bool) (/lib64/libboost_unit_test_framework.so.1.73.0+0x38547)
    scylladb#23 0x7fbf45e43618 in boost::unit_test::unit_test_main(bool (*)(), int, char**) (/lib64/libboost_unit_test_framework.so.1.73.0+0x4d618)
    scylladb#24 0x44798d in seastar::testing::entry_point(int, char**) ../../src/testing/entry_point.cc:77
    scylladb#25 0x4134b5 in main ../../include/seastar/testing/seastar_test.hh:65
    scylladb#26 0x7fbf44a1b1e1 in __libc_start_main (/lib64/libc.so.6+0x281e1)
    scylladb#27 0x4133dd in _start (/home/bhalevy/dev/seastar/build/debug/tests/unit/closeable_test+0x4133dd)
```

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Message-Id: <20210406100911.12278-1-bhalevy@scylladb.com>

Author: bhalevy

tests/unit/closeable_test.cc

@@ -29,10 +29,7 @@
 using namespace seastar;
 
 SEASTAR_TEST_CASE(deferred_close_test) {
-    int count = 0;
-    int expected = 42;
-    gate g;
-
+  return do_with(gate(), 0, 42, [] (gate& g, int& count, int& expected) {
     return async([&] {
         auto close_gate = deferred_close(g);
 
@@ -47,13 +44,11 @@ SEASTAR_TEST_CASE(deferred_close_test) {
         BOOST_REQUIRE(g.is_closed());
         BOOST_REQUIRE_EQUAL(count, expected);
     });
+  });
 }
 
 SEASTAR_TEST_CASE(close_now_test) {
-    int count = 0;
-    int expected = 42;
-    gate g;
-
+  return do_with(gate(), 0, 42, [] (gate& g, int& count, int& expected) {
     return async([&] {
         auto close_gate = deferred_close(g);
 
@@ -68,6 +63,7 @@ SEASTAR_TEST_CASE(close_now_test) {
         BOOST_REQUIRE_EQUAL(count, expected);
         // gate must not be double-closed.
     });
+  });
 }
 
 namespace {
@@ -84,19 +80,18 @@ struct count_stops {
 } // anonymous namespace
 
 SEASTAR_TEST_CASE(deferred_stop_test) {
-    count_stops cs;
-
+  return do_with(count_stops(), [] (count_stops& cs) {
     return async([&] {
         auto stop_counting = deferred_stop(cs);
     }).then([&] {
         // cs.stop() should be called when stop_counting is destroyed
         BOOST_REQUIRE_EQUAL(cs.stopped, 1);
     });
+  });
 }
 
 SEASTAR_TEST_CASE(stop_now_test) {
-    count_stops cs;
-
+  return do_with(count_stops(), [] (count_stops& cs) {
     return async([&] {
         auto stop_counting = deferred_stop(cs);
 
@@ -108,4 +103,5 @@ SEASTAR_TEST_CASE(stop_now_test) {
         // cs.stop() should be called exactly once
         BOOST_REQUIRE_EQUAL(cs.stopped, 1);
     });
+  });
 }