Port Test262’s RegExp lookbehind tests

[mathiasbynens]

This patch ports the tests from https://github.com/tc39/test262/tree/main/test/built-ins/RegExp/lookBehind into WPT.

Any uses of the following Test262 harness APIs have been replaced with the corresponding WPT testharness.js API:

• assert.compareArray → assert_array_equals
• assert.sameValue → assert_equals

The tests pass in Chrome and Firefox.

This is motivated by the proposal to include RegExp lookbehind functionality in Interop 2023, which would require WPT coverage since Test262 results cannot yet be consumed / reported by the wpt.fyi & Interop dashboards.

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[mathiasbynens]

Question to reviewers: stable Safari doesn’t yet support lookbehinds, and throws an early parse error when it encounters the first lookbehind in JS source. As a result, the test harness report page remains blank. Is this acceptable to convey the “fail” state?

(If we wanted the harness to show up, we’d have to turn all RegExp literals using lookbehinds into a string literal passed to the RegExp() constructor within a try-catch, with the catch handling the case of no parsing support.)

[jgraham]

Thanks for doing this!

[foolip]

Question to reviewers: stable Safari doesn’t yet support lookbehinds, and throws an early parse error when it encounters the first lookbehind in JS source. As a result, the test harness report page remains blank. Is this acceptable to convey the “fail” state?

Since it's already fixed in STP I think that's OK, yeah. It's certainly not the best failure more, but good enough for something that's being fixed.