Tweak specs to try to avoid flakiness

I saw some flakiness in CI, so this is a very soft attempt at avoiding
it; let's see if it's enough or we'll need more changes.

spec/datadog/profiling/collectors/cpu_and_wall_time_worker_spec.rb

@@ -549,6 +549,8 @@
 
           thread_that_allocates_as_fast_as_possible = Thread.new { loop { BasicObject.new } }
 
+          Thread.pass
+
           allocation_samples = try_wait_until do
             samples = samples_from_pprof(recorder.serialize!).select { |it| it.values[:'alloc-samples'] > 0 }
             samples if samples.any? { |it| it.labels[:'thread name'] == 'Skipped Samples' }
@@ -557,6 +559,8 @@
           thread_that_allocates_as_fast_as_possible.kill
           thread_that_allocates_as_fast_as_possible.join
 
+          GC.start
+
           cpu_and_wall_time_worker.stop
 
           expect(allocation_samples).to_not be_empty

spec/datadog/profiling/stack_recorder_spec.rb

@@ -399,6 +399,7 @@ def sample_allocation(obj)
       end
 
       before do
+        GC.start
         allocations = [a_string, an_array, "a fearsome interpolated string: #{sample_rate}", (-10..-1).to_a, a_hash,
                        { 'z' => -1, 'y' => '-2', 'x' => false }, Object.new]
         @num_allocations = 0