Change test to use full-path-for-testing attribute selector

This makes this test compatible with directory tree having My Files or
not, and makes the test more readable too.

This is a preparation to enable new navigation (with My Files) by
default.

Bug: 857335
No-presubmit: true
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Ia35bc2cc7c91e0492f0158990bfc397a6b57da21
Reviewed-on: https://chromium-review.googlesource.com/1133610
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574500}
3 files changed