browser.test_wasm_worker_lock_async_acquire is flaky

The test seems to be racy to occassionally postMessage() to a Wasm Worker after the test has finished, resulting in

```
======================================================================
FAIL: test_wasm_worker_lock_async_acquire (test_browser.browser_2gb.test_wasm_worker_lock_async_acquire)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/clb/.pyenv/versions/3.13.3/lib/python3.13/unittest/case.py", line 58, in testPartExecutor
yield
File "/home/clb/.pyenv/versions/3.13.3/lib/python3.13/unittest/case.py", line 651, in run
self._callTestMethod(testMethod)
~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
File "/home/clb/.pyenv/versions/3.13.3/lib/python3.13/unittest/case.py", line 606, in _callTestMethod
if method() is not None:
~~~~~~^^
File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/common.py", line 1049, in resulting_test
return func(self, *args)
File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/common.py", line 668, in metafunc
f(self, *args, **kwargs)
~^^^^^^^^^^^^^^^^^^^^^^^
File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/test_browser.py", line 5253, in test_wasm_worker_lock_async_acquire
self.btest_exit('wasm_worker/lock_async_acquire.c', cflags=['--closure=1', '-sWASM_WORKERS'])
~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/common.py", line 2835, in btest_exit
return self.btest(filename, *args, **kwargs)
~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/common.py", line 2864, in btest
self.run_browser(outfile, expected=['/report_result?' + e for e in expected], timeout=timeout)
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/common.py", line 2787, in run_browser
raise e
File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/common.py", line 2778, in run_browser
self.assertContained(expected, output)
~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/common.py", line 1835, in assertContained
self.fail("Expected to find '%s' in '%s', diff:\n\n%s\n%s" % (
~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
limit_size(values[0]), limit_size(string), limit_size(diff),
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
additional_info,
^^^^^^^^^^^^^^^^
))
^^
File "/home/clb/.pyenv/versions/3.13.3/lib/python3.13/unittest/case.py", line 732, in fail
raise self.failureException(msg)
AssertionError: Expected to find '/report_result?exit:0
' in '/report_result?exception:X[a] is undefined / hb@http://localhost:8916/test.js:543:3
@http://localhost:8916/test.wasm:wasm-function[22]:0x715
@http://localhost:8916/test.wasm:wasm-function[23]:0x816
bb/e/<@http://localhost:8916/test.js:502:12
', diff:

--- expected
+++ actual
@@ -1 +1,5 @@
-/report_result?exit:0
+/report_result?exception:X[a] is undefined / hb@http://localhost:8916/test.js:543:3
+@http://localhost:8916/test.wasm:wasm-function[22]:0x715
+@http://localhost:8916/test.wasm:wasm-function[23]:0x816
+bb/e/<@http://localhost:8916/test.js:502:12
+

----------------------------------------------------------------------
```

In the above, `X[a]` is the `$_wasmWorkers` dictionary in `src/libwasm_workers.js`.