[AsyncSB] Add PostCommitInterstitial browser tests

Previously, several subresource browser tests were disabled because of
kSafeBrowsingSkipSubresources. Some of these tests are in essence
testing the functionality of post commit interstitial and should not
be limited to warnings triggered by subresources.

This CL ports the tests that are relevant to post commit interstitial
under SafeBrowsingBlockingPageAsyncChecksTimingTest. It reveals an
issue that the security indicator is persisted if the Safe Browsing
check is completed between ProcessResponse and FinishNavigation
(see the TODO in SecurityStateGoBackOnPostCommitInterstitial). It
will be fixed in a follow up CL.

The old tests are removed entirely since we plan to clean up the
kSafeBrowsingSkipSubresources flag soon.

This CL also removes the "MAYBE" before
IframeOptInAndReportThreatDetails. We will monitor if this test is still
flaky on Mac.

Bug: 40941453,40934310
Change-Id: I64ea7478bf842ccc726244df5814e6906c120295
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5282249
Reviewed-by: thefrog <thefrog@chromium.org>
Commit-Queue: Xinghui Lu <xinghuilu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1259959}