tests/rpm.t: delete the working directory in case it exists

Change-Id: Iea15d296e22a36b119f04aff61a975811054c20b
BUG: 1092850
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/7916
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>