Move tests under paint/dark-mode/* to virtual/dark-mode*, so that
virtual/dark-mode*/paint/dark-mode/* now becomes
virtual/dark-mode*.

Also we no longer run paint/dark-mode/* test in non-dark mode.

Bug: 1014162
Change-Id: I23b80e4a535356855f11aa2f47a517fbc455f171
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1892214Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711371}