drive: Add tests for FileSystem methods

There are tests for each operation and each sync performer.
But, no test verifies operation classes and performer classes working together.

Add tests for FileSystem::Copy(), Move(), Remove(), CreateDirectory(), CreateFile(), TouchFile() and TruncateFile().

BUG=340210
TEST=unit_tests --gtest_filter="FileSystemTest.*"

Review URL: https://codereview.chromium.org/158723003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252041 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252041