Let iOS bots run unittests with XCTest.

When running Gtest-based tests, we use the WebRtcUnitTestDelegate
to call Gtest from the iOS application.
However, if tests take too long, iOS terminates that application.
The test execution code (ios/build/bots/scripts) retries up to three times,
but that functionality doesn't seem to work lately.
Because of this, module_unittests are failing at a fairly high rate.

Instead, use GoogleTestRunner to let XCTest run Gtest-based tests.
This is enabled with the --enable-run-ios-unittests-with-xctest flag,
which is passed when using the --xctest flag in ios/build/bots/scripts/run.py.
Existing XCTest-based tests (eg. sdk_unittest) are not affected
as the --xcode-parallelization flag takes precedence over --xctest.

Bug: None
Change-Id: Ib7f8a6d24f6b25444a47e3a83c0edbe96318be46
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/287180
Reviewed-by: Jeremy Leconte <jleconte@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Daniel.L (Byoungchan) Lee <daniel.l@hpcnt.com>
Cr-Commit-Position: refs/heads/main@{#38859}