[devtools] Fix flaky ExecutionContext events inspector test

A recent CL changed the timing when ExecutionContexts are destroyed.
For iframes, ExecutionContexts are kept alive even when the iframe
is disconnected from the page. This makes ExecutionContext destruction
timing depended on GC timing.

This CL fixes a related inspector-protocol test by doing the following:
  1) Wait for iframe content to be loaded. Otherwise we attempt to
     remove an iframe that's maybe not there.
  2) Rather than hard coding the order of ExecutionContextDestroyed
     events, we simply wait at the end of the test and make sure that
     all created contexts are also destroyed. It doesn't really
     matter when, as long as they are properly cleaned up.

The test is still rather slow due to all the navigations. Locally,
it hovers around the 6 second runtime mark, causing sporadic
timeouts. This CL also marks the test as "Slow", so it is run
with a larger timeout window.

R=wolfi@chromium.org

Fixed: 1236466
Change-Id: Ieaad8887c7f4e3f4415a59d778218f4ac817b707
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3240967
Reviewed-by: Wolfgang Beyer <wolfi@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#934518}

• third_party/blink/web_tests/SlowTests[diff]
• third_party/blink/web_tests/TestExpectations[diff]
• third_party/blink/web_tests/inspector-protocol/runtime/runtime-execution-contexts-events-expected.txt[diff]
• third_party/blink/web_tests/inspector-protocol/runtime/runtime-execution-contexts-events.js[diff]