`browser.test_pthread_run_on_main_thread` is flaky

[kleisauke]

It looks like browser.test_pthread_run_on_main_thread is quite flaky lately.

Failed CI builds so far of the last 20 days (PRs excluded)

https://circleci.com/gh/emscripten-core/emscripten/490388
https://circleci.com/gh/emscripten-core/emscripten/490741
https://circleci.com/gh/emscripten-core/emscripten/491243
https://circleci.com/gh/emscripten-core/emscripten/491256
https://circleci.com/gh/emscripten-core/emscripten/491569
https://circleci.com/gh/emscripten-core/emscripten/491654
https://circleci.com/gh/emscripten-core/emscripten/491755
https://circleci.com/gh/emscripten-core/emscripten/491794
https://circleci.com/gh/emscripten-core/emscripten/492496
https://circleci.com/gh/emscripten-core/emscripten/492485
https://circleci.com/gh/emscripten-core/emscripten/492811
https://circleci.com/gh/emscripten-core/emscripten/493027
https://circleci.com/gh/emscripten-core/emscripten/493364
https://circleci.com/gh/emscripten-core/emscripten/493582
https://circleci.com/gh/emscripten-core/emscripten/494479
https://circleci.com/gh/emscripten-core/emscripten/494966
https://circleci.com/gh/emscripten-core/emscripten/494967
https://circleci.com/gh/emscripten-core/emscripten/495378
https://circleci.com/gh/emscripten-core/emscripten/495574
https://circleci.com/gh/emscripten-core/emscripten/495819
https://circleci.com/gh/emscripten-core/emscripten/496390
https://circleci.com/gh/emscripten-core/emscripten/496486
https://circleci.com/gh/emscripten-core/emscripten/496914
https://circleci.com/gh/emscripten-core/emscripten/497178
https://circleci.com/gh/emscripten-core/emscripten/497229
https://circleci.com/gh/emscripten-core/emscripten/497363

It seems to hang after Testing async proxied runs:.

emscripten/tests/pthread/test_pthread_run_on_main_thread.cpp

Lines 95 to 103 in 17cbd29

	printf("Testing async proxied runs:\n");
	emscripten_async_run_in_main_runtime_thread(EM_FUNC_SIG_V, v);
	emscripten_async_run_in_main_runtime_thread(EM_FUNC_SIG_VI, vi, 42);
	emscripten_async_run_in_main_runtime_thread(EM_FUNC_SIG_VII, vii, 42, 43);
	emscripten_async_run_in_main_runtime_thread(EM_FUNC_SIG_VIII, viii, 42, 43, 44);
	emscripten_async_run_in_main_runtime_thread(EM_FUNC_SIG_I, i);
	emscripten_async_run_in_main_runtime_thread(EM_FUNC_SIG_II, ii, 42);
	emscripten_async_run_in_main_runtime_thread(EM_FUNC_SIG_III, iii, 42, 43);
	emscripten_async_run_in_main_runtime_thread(EM_FUNC_SIG_IIII, iiii, 42, 43, 44);

This issue was opened to further investigate the behavior of this test.

[kleisauke]

Hopefully the new proxying API (PR #15880) would fix this. Perhaps we should disable this test for now since it fails frequently.