mdadm: skip running known broken ptests

mdadm: skip running known broken ptests

Upstream marked some testcases as "KNOWN BROKEN" and introduced the
"--skip-broken" flag to ignore them when running the testsuite (commits [1]
and [2]). Backport these two commits to get rid of the last remaining ptest
failures.

Also, add the "--skip-broken" option to the run-ptest script.

[1] https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=28520bf114b3
[2] https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=daa86d663476

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>