[Fuchsia] Remove CheckRunWithTimeout() test helper.

The ScopedTaskEnvironment now configures a Run() timeout for the test's
main thread, removing the need for the custom CheckRunWithTimeout()
helper.

Bug: 893236
Change-Id: If002158e986ca279860715b78c6b0ee0b52da811
Reviewed-on: https://chromium-review.googlesource.com/c/1401852
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621401}
7 files changed