Skip whole browser test, not just the execution part (#25426)

I am running browser test suites several times, with

```
set EMCC_CFLAGS=-sMIN_FIREFOX_VERSION=xyz -Wno-unused-command-line-argument
```
but even when I also set the environment variables
```
set EMTEST_LACKS_OFFSCREEN_CANVAS=1
set EMTEST_LACKS_ES6_WORKERS=1
```
to skip the relevant ES6 or OffscreenCanvas tests that the Firefox
browser under test does not support, the test will actually not skip,
but still produce a test failure.

This is because the `skipExecIf()` function used to build the
`@requires_offscreen_canvas` and `@requires_es6_workers` decorators will
still attempt to compile the test, and only skip running it in the
browser.

But when I have set e.g. `EMCC_CFLAGS=-sMIN_FIREFOX_VERSION=95`, then
that Firefox does not support OffscreenCanvas, and it does not help even
if I also set the env. var. `EMTEST_LACKS_OFFSCREEN_CANVAS=1`. The
failing compilation still happens, and the test fails.

This PR fixes that issue by making EMTEST_LACKS_OFFSCREEN_CANVAS and
EMTEST_LACKS_ES6_WORKERS skip the whole test, and not just the browser
execution part.

Though this does have the drawback that CircleCI coverage might be
reduced, e.g. for the OffscreenCanvas tests parts, if it was seen
valuable to still compile those on targets that don't support
OffscreenCanvas. But is that important anymore?

Author: juj

test/test_browser.py

@@ -202,8 +202,8 @@ def webgpu_disabled():
 requires_webgl2 = unittest.skipIf(webgl2_disabled(), "This test requires WebGL2 to be available")
 requires_webgpu = unittest.skipIf(webgpu_disabled(), "This test requires WebGPU to be available")
 requires_sound_hardware = skipExecIf(os.getenv('EMTEST_LACKS_SOUND_HARDWARE'), 'This test requires sound hardware')
-requires_offscreen_canvas = skipExecIf(os.getenv('EMTEST_LACKS_OFFSCREEN_CANVAS'), 'This test requires a browser with OffscreenCanvas')
-requires_es6_workers = skipExecIf(os.getenv('EMTEST_LACKS_ES6_WORKERS'), 'This test requires a browser with ES6 Module Workers support')
+requires_offscreen_canvas = unittest.skipIf(os.getenv('EMTEST_LACKS_OFFSCREEN_CANVAS'), 'This test requires a browser with OffscreenCanvas')
+requires_es6_workers = unittest.skipIf(os.getenv('EMTEST_LACKS_ES6_WORKERS'), 'This test requires a browser with ES6 Module Workers support')
 
 
 class browser(BrowserCore):