fix(wpt): run async tests and collect results

[huancheng-trili]

Context

Closes JSTZ-256.
JSTZ-256

Description

Fixes the wpt runner in jstz_api so that it covers async test cases. Asynchronous tests, including those wrapped in promises, were not properly executed because eval only runs the synchronous tests and puts async ones in the task queue. The runtime task queue was never touched afterwards and thus the async tests were simply abandoned. With changes in this PR, async test cases can be executed as expected, and also because of this, we don't need the Null test status because at the end of the async task chain, test suites are completed and therefore there is always a meaningful test status returned.

Note that the task queue must be executed after all synchronous tests finish, i.e. after all eval calls are made. I haven't fully figured out why, but my guess is that the test harness API has some assumptions about the test flow for each test suite, e.g. after async tests are done, other things are ignored, such that some test cases are dropped if run_jobs is called right after an eval call.

Manually testing the PR

env UPDATE_EXPECT=1 cargo test --test wpt

and more tests show up in wptreport.json. These tests are already defined in the test suites but they were never executed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.87%. Comparing base (373ec79) to head (dadfa5c).
Report is 1 commits behind head on main.

Files with missing lines	Coverage Δ
crates/jstz_wpt/src/report.rs	0.00% <ø> (ø)

... and 1 file with indirect coverage changes

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[huancheng-trili]

@johnyob do you have any other comments? I added some notes in wpt.rs explaining the use of unwrap and Option.