flaky test failure with ruby3.1

[rrrrrrmb]

Running the tests with ruby3.1 seems to produce the following failure.Sometimes the tests randomly passes.

/usr/bin/ruby3.1 -w -I"test" /usr/lib/ruby/gems/3.1.0/gems/rake-13.0.6/lib/rake/rake_test_loader.rb "test/test_middleware.rb" "test/test_report.rb" "test/test_stackprof.rb" "test/test_truffleruby.rb" -v
Run options: -v --seed 49192
# Running:
StackProfTest#test_metadata = 0.01 s = .
StackProfTest#test_object_allocation = 0.00 s = .
StackProfTest#test_gc = 0.02 s = F
StackProfTest#test_info = 0.00 s = .
StackProfTest#test_min_max_interval = 0.00 s = .
StackProfTest#test_pathname_out = 0.00 s = .
StackProfTest#test_raises_if_metadata_is_not_a_hash = 0.00 s = .
StackProfTest#test_start_stop_results = 0.00 s = .
StackProfTest#test_custom = 0.00 s = .
StackProfTest#test_cputime = 0.01 s = .
StackProfTest#test_running = 0.00 s = .
StackProfTest#test_fork = 0.00 s = .
StackProfTest#test_empty_metadata = 0.01 s = .
StackProfTest#test_object_allocation_interval = 0.00 s = .
StackProfTest#test_out = 0.00 s = .
StackProfTest#test_out_to_path_string = 0.00 s = .
StackProfTest#test_recursive_total_samples = 0.00 s = .
StackProfTest#test_raw = 0.00 s = .
StackProfTest#test_walltime = 0.20 s = .
StackProf::MiddlewareTest#test_enabled_should_use_a_proc_if_passed = 0.00 s = .
StackProf::MiddlewareTest#test_metadata = 0.00 s = .
StackProf::MiddlewareTest#test_path_default = 0.00 s = .
StackProf::MiddlewareTest#test_save_custom = 0.00 s = .
StackProf::MiddlewareTest#test_path_custom = 0.00 s = .
StackProf::MiddlewareTest#test_raw = 0.00 s = .
StackProf::MiddlewareTest#test_enabled_should_use_a_proc_if_passed_and_use_the_request_env = 0.00 s = .
StackProf::MiddlewareTest#test_save_default = 0.00 s = .
ReportDumpTest#test_dump_to_file = 0.02 s = .
ReportDumpTest#test_dump_to_stdout = 0.00 s = .
ReportReadTest#test_from_file_read_marshal = 0.00 s = .
ReportReadTest#test_from_file_read_json = 0.00 s = .
Finished in 0.293901s, 105.4778 runs/s, 445.7288 assertions/s.
  1) Failure:
StackProfTest#test_gc [/builds/ruby-team/ruby-stackprof/debian/output/source_dir/test/test_stackprof.rb:242]:
Expected nil to be truthy.
31 runs, 131 assertions, 1 failures, 0 errors, 0 skips
rake aborted!