WPT: Allow window.onload to contain multiple test()s

[chromium-wpt-export-bot]

---

*** SHERIFFS: please don't revert this CL if it causes web_tests
to flake/fail. If that happens, the cause is a bad
test. Please mark that test as flaky/fail in
TestExpectations, with a new crbug. Please block the
new bug against crbug.com/1395228. Thanks!

---

Prior to this CL, a test like this:

\<script>
window.onload = () => {
  test((t) => { ... }, 'test 1');
  test((t) => { ... }, 'test 2');
  test((t) => { ... }, 'test 3');
};
\</script>

would not run anything after test #1. The issue is that the testharness
immediately adds a window load handler that marks all_loaded = true,
and that ends the tests as soon as the first result from the first test
is processed. (The test runner waits for the first test because
Tests.prototype.all_done() also waits until this.tests.length > 0.)
There were various mitigating corner cases, such as if you started
the list of tests with a promise_test(), that would increment a
counter that kept the rest of the tests alive. Etc.

With this CL, the testharness-added window.onload handler runs a
setTimeout(0), so that all_loaded is only set to true after all of
the tests are loaded by any window.onload handler.

This exposed a few tests that should have been failing but were
masked by the lack of test coverage - bugs have been filed for
those. Also, several tests that were working around this via various
means are also cleaned up in this CL. I'm sure there are more of
those.

Bug: 1395228,1395226,1307772
Change-Id: I6f12b5922186af4e1e06808ad23b47ceac68559c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4074305
Reviewed-by: Weizhong Xia <weizhong@google.com>
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1081558}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.