tests/basic/rpm.t: run mock builds in parallel

save save about five minutes per regression

Change-Id: I75d91cfef63347ca756b9e1fedcb697e9537edc0
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
Reviewed-on: http://review.gluster.org/5828
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>