Call parent::setUp() in DiffHistoryBlobTest before marking skipped tests

Jenkins skips these tests because xdiff is unavailable. The tests get
marked as skipped, out of order and before the error reporting setup.

Things are left in an inconsistent state when running *all* the tests together,
and interferes with later error handling (including PHPUnit_Framework_Error)
in other tests.

The gate-and-submit and other jobs are split into multiple jobs,
and for some reason does not trigger this issue.

This will probably fix bug 57488.

Bug: 57488
Change-Id: Ie1a213e912b5858b43956fc35cf00226636f2093
Merge "Call parent::setUp() in DiffHistoryBlobTest before marking skipped tests"