Fix passing MIN_x_VERSION in EMCC_CFLAGS when running with a skip env. var. set. (#25753)

This is a bit of awkward combination of things.

On my CI, I test old Firefox (and Safari) browsers down to the min-spec,
e.g. Firefox 68, Firefox 78, Firefox 91, and so on.

But because the default MIN_x_VERSION in src/settings.js is newer than
the supported min-spec, I need to pass env. var.
`EMCC_CFLAGS=-sMIN_x_VERSION=68` for example when running the browser
test suite.

I combine that env. var. with the new `EMTEST_AUTOSKIP=1` env. var. so
that I can offload having to maintain a large knowledgebase of which
browser supports/doesn't support which feature inside the test runner
itself.

This works great - almost.

The issue is that the browser feature skipping mechanism
(`EMTEST_AUTOSKIP` and `EMTEST_LACKS_x` env. vars) has an intentional
feature of skipping **only** the browser test run part, but not the test
execution/Emscripten compilation part.

The intent of this mechanism has been to be able to partially test the
browser tests, even if a skip env. var. is present, but just not run the
test in an old browser that's known to fail.

But this results in the problem. When one uses e.g.

```
EMTEST_BROWSER=/path/to/firefox-91/firefox
EMCC_CFLAGS=-sMIN_FIREFOX_VERSION=91
EMTEST_LACKS_ES6_WORKERS=1
```

then the test `browser.test_audio_worklet_es6` will not skip, but will
error out with a feature matrix check
```
emcc: error: MIN_FIREFOX_VERSION=91 is not compatible with EXPORT_ES6 with -sWASM_WORKERS (MIN_FIREFOX_VERSION=114 or above required) [-Wcompatibility] [-Werror]
```

To fix this, we could either change the semantics of the skip env. vars
to always skip the whole test, not just the running part. But that might
reduce test coverage that was intentional to preserve.

Or we could adjust the `EMTEST_AUTOSKIP=1` env. var. to be special and
have a different "power level" of skipping compared to the
`EMTEST_LACKS_x=1` env. vars. But that feels arbitrary and prone to
future confusion.

So this PR adds a bit of an awkward test to skip the whole test if any
MIN_x_VERSION directives are explicitly present in EMCC_CFLAGS env. var.
This way the above combination of flags will work to skip as desired,
but will not reduce the test coverage on CircleCI.

Author: juj

test/test_browser.py

@@ -6,6 +6,7 @@
 import argparse
 import os
 import random
+import re
 import shlex
 import shutil
 import subprocess
@@ -236,8 +237,15 @@ def decorator(f):
     def decorated(self, *args, **kwargs):
       if should_skip == 'error':
         raise Exception(f'This test requires a browser that supports {feature.name} but your browser {get_browser()} does not support this. Run with {skip_env_var}=1 or EMTEST_AUTOSKIP=1 to skip this test automatically.')
+      elif should_skip and bool(re.search(r"MIN_.*_VERSION", os.getenv('EMCC_CFLAGS', ''))):
+        # should_skip=True, so we should skip running this test. However, user has specified a MIN_x_VERSION
+        # directive in EMCC_CFLAGS, so we cannot even try to compile this test, or otherwise emcc can
+        # error out on the MIN_x_VERSION being too old. So skip both compiling+running this test.
+        self.skipTest(message)
       elif should_skip:
+        # Skip running this test in a browser, but do test compiling it, to get partial coverage.
         self.skip_exec = message
+
       f(self, *args, **kwargs)
 
     return decorated