Meta+headless-browser: Add support for crash tests

[tcl3]

This PR adds support for both our own internal crash tests and allows us to import crash tests from WPT. These are essentially the same as text tests, but they have no associated expectation file. These tests pass if the test fully loads without crashing.

NOTE: The WPT runner uses WebDriver to execute this script when running crash tests. We don't currently do this, which may lead to some imported tests finishing before they've been given the opportunity to crash. I haven't included an equivalent mechanism in this PR, as it complicates the import processes and didn't seem necessary for some crash tests I tried to import while testing.

Advances: #2282