Cleanup mojo tests

TestSupportingGC finishes sweeping on destruction.
No need for mojo tests to explicitly finish.

Bug: 1056170
Change-Id: I98225c9ae587577692ee51a1ffe486472edde78c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2797580


Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#868515}