[testing] Move context menu ever timestamp test to a separate test

ContextMenu interaction with other events is a suspected cause of
flakiness of the original test. This CL is separating it from the
rest as a potential fix.

Bug: 846750
Change-Id: I3d452388be65984ca82238d55a938d7a273fcf3f
Reviewed-on: https://chromium-review.googlesource.com/1134180
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574573}
2 files changed