[Files app] Add context menu test for USB with DCIM folder

Re-sending CL: 1514005 based in the new strategy to make it non-flaky.

Add more explicit waiting on checkContextMenu to wait for each entry to
be displayed before trying to expand or click.

Change-Id: Id7a6f9ea27d5bade800837480df79721dcab4329
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524749
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641087}
2 files changed