sparc: disable qtest in make check

We've seen this repeatedly in buildbot but I can now reliably
reproduce it myself too.  With a few hundred runs of 'make check',
qemu-system-sparc will hang consuming 100% CPU.  I've attached GDB
to the hung process and unfortunately, I can't get anything useful
out of GDB (RIP is not a valid simple and there is nothing else on
the stack).

At any rate, since this only manifests in qemu-system-sparc and it
doesn't appear to be a qtest specific problem, I think we should
disable it until the problem is resolved.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
