Add support for WPT test variants, which allow a single test file to be
run multiple times with different URL query parameters. Tests declare
variants using `<meta name="variant" content="?param=value">` tags.
When test-web encounters a test with variants, it expands that test into
multiple runs, each with its own expectation file using the naming
convention `testname@variant.txt` (e.g., `test@run_type=uri.txt`).
Implementation details:
- WebContent observes variant meta tags and communicates them to the
test runner via a new `did_receive_test_variant_metadata` IPC call
- test-web dynamically expands tests with variants during execution,
waking idle views after each test completion to pick up new work
- Use index-based test tracking to avoid dangling references when the
test vector grows during variant expansion
- Introduce TestRunContext to group test run state, and store a static
pointer to it for signal handler access
This enables proper testing of WPT tests that use variants, such as the
html5lib parsing tests (which test uri, write, and write_single modes)
and the editing/bold tests (which split across multiple ranges).
- Add --results-dir CLI flag to specify output directory
- Default to {tempdir}/test-web-results if not specified
- Capture stdout/stderr from all helper processes (WebContent,
RequestServer, ImageDecoder) to prevent output spam
- Save captured output to per-test files in results directory
- Save test diffs (expected vs actual) to results directory
- Generate HTML index of failed tests with links to diffs
- Display live-updating concurrent test status with progress bar
- Defer warning messages until after test run completes
Clearing the callback opens a window for the WebContent process to crash
while we do not have a callback set. In practice, we see this with ASAN
crashes, where the crash occurs after we've already signaled that the
test has completed.
We now set the crash handler once during init. This required moving the
clearing of other callbacks to the test completion handler (we were
clearing these callbacks in several different ways anyways, so now we
will at least be consistent).
Now that headless mode is built into the main Ladybird executable, the
headless-browser's only purpose is to run tests. So let's move it to the
testing directory and rename it to test-web (a la test-js / test-wasm).
