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
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).
