[Files app] Fix PathUtil unittest with MyFiles enabled

Move tests specific for Downloads that are only relevant with
MyFilesVolume disable to its own scope with this flag disabled. When the
flag is removed these tests can be converted to take into account
/MyFiles path properly.

Bug: 873539
Change-Id: I0052154810fd9681bf277928db450d656af653da
Reviewed-on: https://chromium-review.googlesource.com/c/1428002
Commit-Queue: Sam McNally <sammc@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625030}
1 file changed