[NO-TICKET] Switch libdatadog-crashtracking-receiver checks to wait_for

**What does this PR do?**

This PR changes the crashtracking specs to use `wait_for` when asserting
on the `libdatadog-crashtracking-receiver` process still being alive or
not.

**Motivation:**

While experimenting with moving our test suite to GitHub Actions, this
seemed to trigger a lot.

Also, we saw it trigger at least once for CircleCI as well:
https://app.circleci.com/pipelines/github/DataDog/dd-trace-rb/15402/workflows/d3d6f9ad-7d9f-4845-9f6b-bc92eaf88b74/jobs/563014

**Additional Notes:**

Fixes DataDog/ruby-guild#176 .

I'll see if there's something we can improve on the libdatadog side
to avoid this flakiness, but let's not live with the flaky specs until
we do so.

**How to test the change?**

Validate that CI is still green.

spec/datadog/profiling/crashtracker_spec.rb

@@ -8,8 +8,13 @@
   before do
     skip_if_profiling_not_supported(self)
 
-    crash_tracker_pids = `pgrep -f libdatadog-crashtracking-receiver`
-    expect(crash_tracker_pids).to be_empty, "No crash tracker process should be running, found #{crash_tracker_pids}"
+    # No crash tracker process should still be running at the start of each testcase
+    wait_for { `pgrep -f libdatadog-crashtracking-receiver` }.to be_empty
+  end
+
+  after do
+    # No crash tracker process should still be running at the end of each testcase
+    wait_for { `pgrep -f libdatadog-crashtracking-receiver` }.to be_empty
   end
 
   let(:exporter_configuration) { [:agent, 'http://localhost:6006'] }
@@ -59,7 +64,7 @@
     it 'starts the crash tracker' do
       start
 
-      expect(`pgrep -f libdatadog-crashtracking-receiver`).to_not be_empty
+      wait_for { `pgrep -f libdatadog-crashtracking-receiver` }.to_not be_empty
 
       crashtracker.stop
     end
@@ -68,7 +73,7 @@
       it 'only starts the crash tracker once' do
         3.times { crashtracker.start }
 
-        expect(`pgrep -f libdatadog-crashtracking-receiver`.lines.size).to be 1
+        wait_for { `pgrep -f libdatadog-crashtracking-receiver`.lines.size }.to be 1
 
         crashtracker.stop
       end
@@ -95,13 +100,15 @@
 
       it 'starts a second crash tracker for the fork' do
         expect_in_fork do
+          wait_for { `pgrep -f libdatadog-crashtracking-receiver`.lines.size }.to be 1
+
           crashtracker.reset_after_fork
 
-          expect(`pgrep -f libdatadog-crashtracking-receiver`.lines.size).to be 2
+          wait_for { `pgrep -f libdatadog-crashtracking-receiver`.lines.size }.to be 2
 
           crashtracker.stop
 
-          expect(`pgrep -f libdatadog-crashtracking-receiver`.lines.size).to be 1
+          wait_for { `pgrep -f libdatadog-crashtracking-receiver`.lines.size }.to be 1
         end
       end
     end
@@ -124,7 +131,7 @@
 
       stop
 
-      expect(`pgrep -f libdatadog-crashtracking-receiver`).to be_empty
+      wait_for { `pgrep -f libdatadog-crashtracking-receiver` }.to be_empty
     end
   end