qa: fix mount directory

qa: fix mount directory

Otherwise we get this error:

	2020-10-30T03:57:44.256 INFO:teuthology.run_tasks:Running task exec...
	2020-10-30T03:57:44.265 INFO:teuthology.task.exec:Executing custom commands...
	2020-10-30T03:57:44.266 INFO:teuthology.task.exec:Running commands on role client.0 host ubuntu@smithi095.front.sepia.ceph.com
	2020-10-30T03:57:44.266 INFO:teuthology.orchestra.run.smithi095:> sudo TESTDIR=/home/ubuntu/cephtest bash -c 'cd $TESTDIR/mnt.* && dd if=/dev/zero of=./foo count=100'
	2020-10-30T03:57:44.380 INFO:teuthology.orchestra.run.smithi095.stderr:bash: line 0: cd: too many arguments

From: /ceph/teuthology-archive/pdonnell-2020-10-30_02:26:51-fs-master-distro-basic-smithi/5573179/teuthology.log

Fixes: https://tracker.ceph.com/issues/23718
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>