auth: Skip flaky DiskTokenCache test.
auth: Skip flaky DiskTokenCache test.

The source of flakiness if understood, but there's no easy fix that won't
noticeably increase the test runtime.

R=a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org

Change-Id: I1912049cfd0b617f032a006e944da266c0b93a99
Reviewed-on: https://chromium-review.googlesource.com/748421
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go