Audio tests are flaky under firefox headless in CI

Since enabled audio tests in FF headless more in CI in #23701 we have been seeing flakes like this:

```
======================================================================
FAIL [184.888s]: test_sdl_audio_beep_sleep (test_browser.browser)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/root/project/test/common.py", line 2355, in run_browser
self.assertContained(expected, output)
File "/root/project/test/common.py", line 1653, in assertContained
self.fail("Expected to find '%s' in '%s', diff:\n\n%s\n%s" % (
File "/usr/lib/python3.8/unittest/case.py", line 753, in fail
raise self.failureException(msg)
AssertionError: Expected to find '/report_result?exit:0
' in '[no http server activity]
', diff:

--- expected
+++ actual
@@ -1 +1 @@
-/report_result?exit:0
+[no http server activity]

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/root/project/test/common.py", line 896, in resulting_test
return func(self, *args)
File "/root/project/test/test_browser.py", line 216, in decorated
f(self, *args, **kwargs)
File "/root/project/test/test_browser.py", line 3352, in test_sdl_audio_beep_sleep
self.btest_exit('test_sdl_audio_beep_sleep.cpp', emcc_args=['-Os', '-sASSERTIONS', '-sDISABLE_EXCEPTION_CATCHING=0', '-profiling', '-lSDL', '-sASYNCIFY'] + args, timeout=90)
File "/root/project/test/common.py", line 2406, in btest_exit
return self.btest(filename, *args, **kwargs)
File "/root/project/test/common.py", line 2439, in btest
self.run_browser(url, expected=['/report_result?' + e for e in expected], timeout=timeout, extra_tries=extra_tries)
File "/root/project/test/common.py", line 2360, in run_browser
return self.run_browser(html_file, expected, message, timeout, extra_tries - 1)
File "/root/project/test/common.py", line 2362, in run_browser
raise e
File "/root/project/test/common.py", line 2355, in run_browser
self.assertContained(expected, output)
File "/root/project/test/common.py", line 1653, in assertContained
self.fail("Expected to find '%s' in '%s', diff:\n\n%s\n%s" % (
AssertionError: Expected to find '/report_result?exit:0
' in '[no http server activity]
', diff:

--- expected
+++ actual
@@ -1 +1 @@
-/report_result?exit:0
+[no http server activity]

======================================================================
FAIL [186.825s]: test_sdl_audio_beep_sleep_safeheap (test_browser.browser)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/root/project/test/common.py", line 2355, in run_browser
self.assertContained(expected, output)
File "/root/project/test/common.py", line 1653, in assertContained
self.fail("Expected to find '%s' in '%s', diff:\n\n%s\n%s" % (
File "/usr/lib/python3.8/unittest/case.py", line 753, in fail
raise self.failureException(msg)
AssertionError: Expected to find '/report_result?exit:0
' in '[no http server activity]
', diff:

--- expected
+++ actual
@@ -1 +1 @@
-/report_result?exit:0
+[no http server activity]

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/root/project/test/common.py", line 896, in resulting_test
return func(self, *args)
File "/root/project/test/test_browser.py", line 216, in decorated
f(self, *args, **kwargs)
File "/root/project/test/test_browser.py", line 3352, in test_sdl_audio_beep_sleep
self.btest_exit('test_sdl_audio_beep_sleep.cpp', emcc_args=['-Os', '-sASSERTIONS', '-sDISABLE_EXCEPTION_CATCHING=0', '-profiling', '-lSDL', '-sASYNCIFY'] + args, timeout=90)
File "/root/project/test/common.py", line 2406, in btest_exit
return self.btest(filename, *args, **kwargs)
File "/root/project/test/common.py", line 2439, in btest
self.run_browser(url, expected=['/report_result?' + e for e in expected], timeout=timeout, extra_tries=extra_tries)
File "/root/project/test/common.py", line 2360, in run_browser
return self.run_browser(html_file, expected, message, timeout, extra_tries - 1)
File "/root/project/test/common.py", line 2362, in run_browser
raise e
File "/root/project/test/common.py", line 2355, in run_browser
self.assertContained(expected, output)
File "/root/project/test/common.py", line 1653, in assertContained
self.fail("Expected to find '%s' in '%s', diff:\n\n%s\n%s" % (
AssertionError: Expected to find '/report_result?exit:0
' in '[no http server activity]
', diff:

--- expected
+++ actual
@@ -1 +1 @@
-/report_result?exit:0
+[no http server activity]
```