[Extensions] handle exceptions in async tests

If an exception is thrown in an async test, it is not caught, and the
test failure is not logged, like with synchronous functions. Handle the
exception in the "catch()" block if the test returns a promise.

Bug: 1373331
Change-Id: I083295c82e5543c6dc590f8637f089e13bf46f86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3944530
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Alexander Bolodurin <alexbn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1059763}