Flake in test_pthread_attr_init_3_1

https://app.circleci.com/pipelines/github/emscripten-core/emscripten/17445/workflows/948b5972-3b7d-4a35-b53a-e9c0ccdb7fec/jobs/481368

```
======================================================================
FAIL: test_pthread_attr_init_3_1 (test_posixtest.posixtest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/usr/lib/python3.6/unittest/case.py", line 59, in testPartExecutor
yield
File "/usr/lib/python3.6/unittest/case.py", line 605, in run
testMethod()
File "/root/project/tests/common.py", line 197, in decorated
f(self, *args, **kwargs)
File "/root/project/tests/test_posixtest.py", line 166, in f
self.do_runf(testfile, emcc_args=args, output_basename=name)
File "/root/project/tests/common.py", line 1040, in do_runf
return self._build_and_run(filename, expected_output, **kwargs)
File "/root/project/tests/common.py", line 1097, in _build_and_run
interleaved_output=interleaved_output)
File "/root/project/tests/common.py", line 701, in run_js
self.fail('JS subprocess failed (%s): %s. Output:\n%s' % (error.cmd, error.returncode, ret))
File "/usr/lib/python3.6/unittest/case.py", line 670, in fail
raise self.failureException(msg)
AssertionError: JS subprocess failed (/root/emsdk/node/14.15.5_64bit/bin/node --stack-trace-limit=50 --unhandled-rejections=throw --trace-uncaught --experimental-wasm-threads --experimental-wasm-bulk-memory /tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js): 1. Output:
Test PASSED
exiting due to exception: RuntimeError: memory access out of bounds,RuntimeError: memory access out of bounds
at free (:wasm-function[136]:0x476f)
at _emscripten_thread_free_data (:wasm-function[52]:0x1d5b)
at /tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:1740:22
at /tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:2106:11
at Object.runWithoutMainThreadQueuedCalls (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:2116:11)
at Object.returnWorkerToPool (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:2099:17)
at Object.terminateAllThreads (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:2077:21)
at exitRuntime (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:1509:11)
at exit (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:3744:5)
at callMain (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:3642:5)
at doRun (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:3700:23)
at run (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:3715:5)
at runCaller (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:3615:19)
at removeRunDependency (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:1641:7)
at /tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:1831:34
at Worker.worker.onmessage (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:2162:36)
at Worker. (/tmp/tmpfh2ns8nu/emscripten_test_posixtest_awadc678/test_pthread_attr_init_3_1.js:2198:20)
at Worker.emit (events.js:315:20)
at MessagePort. (internal/worker.js:207:53)
at MessagePort.[nodejs.internal.kHybridDispatch] (internal/event_target.js:354:41)
at MessagePort.exports.emitMessage (internal/per_context/messageport.js:18:26)
```