Regression test portability: mock

After some attempts at using mock on NetBSD which pulled a lot of
dependencies, it seems that software really assumes the OS is
Linux: chrooted build will fail because of missing Linux files.

As a result, make tests/basic/rpm.t Linux-only, and remove mock
and rpmbuild checks for non Linux systems.

BUG: 1129939
Change-Id: Ica2eb74dd23fbec379a26051a8f61b0dfc07a115
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8949
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>