Deflake DirectoryTreeContextMenu tests

Change flake tests to use expandTreeItemInDirectoryTree and fix it to
wait for the tree-item being expanded to have "has-children" attribute,
so the click to expand actually works.

Change expandTreeItemInDirectoryTree to use |waitAndClickElement| and
to do so, move it from RemoteCallGallery to the RemoteCall base class.
Change its callbacks from anonymous function to arrow function.

Enable dirRenameWithoutChangingCurrent to run with MyFilesVolume flag
enabled.

Bug: 911609
Change-Id: I10387aef5f213ac2c39f253bf041a5003a2cde17
Reviewed-on: https://chromium-review.googlesource.com/c/1364932
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614584}
3 files changed