wasm64l.test_pthread_dlopen_many is very flakey on github CI

Fails on `JavaScript-side Wasm function table mirror is out of date!`

E.g.:

https://app.circleci.com/pipelines/github/emscripten-core/emscripten/26159/workflows/8150dbce-7431-4fef-96dc-526039140593/jobs/607906

```
[..lots of logging..]
done thread_main 7
jslib_func via dlsym: 0x56
loading liblib9.so
jslib_func via dlsym: 0x56
loading liblib1.so
side module ctor
done loading liblib2.so (total=8)
Aborted(Assertion failed: JavaScript-side Wasm function table mirror is out of date!)
Error in loading dynamic library liblib2.so: RuntimeError: Aborted(Assertion failed: JavaScript-side Wasm function table mirror is out of date!)
_dlopen_js failed: Could not load dynamic lib: liblib2.so
RuntimeError: Aborted(Assertion failed: JavaScript-side Wasm function table mirror is out of date!)
Aborted(Assertion failed: Missing signature argument to addFunction: l)
Aborted(native code called abort())
worker.js onmessage() captured an uncaught exception: RuntimeError: Aborted(Assertion failed: Missing signature argument to addFunction: l)
RuntimeError: Aborted(Assertion failed: Missing signature argument to addFunction: l)
at abort (/tmp/emtest_ptm0hgsk/emscripten_test_wasm64l_y1frjb4q/test_pthread_dlopen_many.js:988:11)
at assert (/tmp/emtest_ptm0hgsk/emscripten_test_wasm64l_y1frjb4q/test_pthread_dlopen_many.js:420:5)
at addFunction (/tmp/emtest_ptm0hgsk/emscripten_test_wasm64l_y1frjb4q/test_pthread_dlopen_many.js:4524:9)
at eval (/tmp/emtest_ptm0hgsk/emscripten_test_wasm64l_y1frjb4q/test_pthread_dlopen_many.js:5255:20)
at __dlsym_catchup_js (/tmp/emtest_ptm0hgsk/emscripten_test_wasm64l_y1frjb4q/test_pthread_dlopen_many.js:5257:8)
at _emscripten_dlsync_self (:wasm-function[63]:0x1a9f)
at do_thread_sync (:wasm-function[66]:0x1c09)
at call_then_schedule_callback (:wasm-function[147]:0x3e40)
at em_task_queue_execute (:wasm-function[122]:0x3678)
at emscripten_proxy_execute_queue (:wasm-function[135]:0x3a2a)
at _emscripten_process_dlopen_queue (:wasm-function[64]:0x1b52)
at emscripten_futex_wait (:wasm-function[82]:0x2035)
at __timedwait_cp (:wasm-function[164]:0x429f)
at __timedwait (:wasm-function[165]:0x4307)
at __pthread_mutex_timedlock (:wasm-function[199]:0x5144)
at __pthread_mutex_lock (:wasm-function[197]:0x5036)
at do_write_lock (:wasm-function[72]:0x1d9c)
at _dlopen (:wasm-function[71]:0x1d02)
at dlopen (:wasm-function[70]:0x1cc4)
at thread_main (:wasm-function[37]:0xf9f)
at eval (/tmp/emtest_ptm0hgsk/emscripten_test_wasm64l_y1frjb4q/test_pthread_dlopen_many.js:4247:52)
at Object.invokeEntryPoint (/tmp/emtest_ptm0hgsk/emscripten_test_wasm64l_y1frjb4q/test_pthread_dlopen_many.js:4247:75)
at handleMessage (/tmp/emtest_ptm0hgsk/emscripten_test_wasm64l_y1frjb4q/test_pthread_dlopen_many.worker.js:169:35)
at MessagePort. (/tmp/emtest_ptm0hgsk/emscripten_test_wasm64l_y1frjb4q/test_pthread_dlopen_many.worker.js:24:38)
at MessagePort.[nodejs.internal.kHybridDispatch] (internal/event_target.js:399:24)
at MessagePort.exports.emitMessage (internal/per_context/messageport.js:18:26)
/tmp/emtest_ptm0hgsk/emscripten_test_wasm64l_y1frjb4q/test_pthread_dlopen_many.js:140
throw ex;
^
```