selftests/powerpc: Skip tm-unavailable if TM is not enabled

Some processor revisions do not support transactional memory, and
additionally kernel support can be disabled. In either case the
tm-unavailable test should be skipped, otherwise it will fail with
a SIGILL.

That commit also sets this selftest to be called through the test
harness as it's done for other TM selftests.

Finally, it avoids using "ping" as a thread name since it's
ambiguous and can be confusing when shown, for instance,
in a kernel backtrace log.

Fixes: 77fad8bf

 ("selftests/powerpc: Check FP/VEC on exception in TM")
Signed-off-by: Gustavo Romero <gromero@linux.vnet.ibm.com>
Reviewed-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>