FilesApp UI test: attempt to fix checkselect flaking

Check #gear-menu select-all for disabled rather than hidden.
Change messages to make exact point of failure clear.

Change-Id: Ie114bdcba79ca0b939d0ea54865961ea1a39b7cd
Reviewed-on: https://chromium-review.googlesource.com/c/1347632
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610277}
1 file changed