[Files F2] Re-enable DirectoryTreeContainer unit test

* Add a debug log to see why it fails starting from Nov 20 on
  linux-chromeos-dbg.
* Remove unnecessary callback in unit tests.
* Clear store before each test.
* Expand Google Drive before checking its children items.

Bug: b:312121966
Test: browser_tests --gtest_filter="*DirectoryTreeContainer"
Cq-Include-Trybots: luci.chromium.try:linux-chromeos-dbg
Change-Id: Ice152d215b78c36986b9f845555e9c7f7ba696df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5058373
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Wenbo Jie <wenbojie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1230413}