tests/basic/rpm.t: run mock builds inside the current working directory

When running multiple mock builds at the same time, the space
requirements under /var/tmp increases as well. Not all systems (like
build.gluster.org) have enough free space in /var/tmp to host two
mock-roots, which causes building to fail randomly.

Change-Id: Iff1064d01893fb35ef59650d4373105068b5d3f7
BUG: 1006269
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/5899
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>