xfstests: change directory to / before _cleanup_testdir in test 135

xfstests: change directory to / before _cleanup_testdir in test 135

Nfs tries to umount $testdir in _cleanup_testdir function. Tests 126
and 135 call the function from directory $SCRATCH_MNT that is equal
to $testdir (at least for nfs). The umount will therefore fail,
causing the test to fail due to the output mismatch.

Test 126 also does double a umount thanks to the call to _cleanup
before exit and the trap command.  So remove the unnecessary call of
the _cleanup function before exit.

Signed-off-by: Boris Ranto <branto@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>