mdadm: improve the run-ptest

mdadm: improve the run-ptest

* There are 120+ cases under ${libdir}/mdadm/ptest/tests,
  but the test will break if one test fails as
  below logic in run-ptest.
  ./test &>./test.log

  That's to say, the tests after the failed test
  have no chance to run with the current logic.

  To guarantee all the tests can run even one
  of the tests fails, the option --keep-going
  should be added.

* Refactor the test report to make the report
  more detailed and more common

Signed-off-by: Mingli Yu <Mingli.Yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>