kunit: tool: make parser stop overwriting status of suites w/ no_tests

Consider this invocation
$ ./tools/testing/kunit/kunit.py parse <<EOF
  TAP version 14
  1..2
  ok 1 - suite
    # Subtest: no_tests_suite
    # catastrophic error!
  not ok 1 - no_tests_suite
EOF

It will have a 0 exit code even though there's a "not ok".

Consider this one:
$ ./tools/testing/kunit/kunit.py parse <<EOF
  TAP version 14
  1..2
  ok 1 - suite
  not ok 1 - no_tests_suite
EOF

It will a non-zero exit code.

Why?
We have this line in the kunit_parser.py
> parent_test = parse_test_header(lines, test)
where we have special handling when we see "# Subtest" and we ignore the
explicit reported "not ok 1" status!

Also, NO_TESTS at a suite-level only results in a non-zero status code
where then there's only one suite atm.

This change is the minimal one to make sure we don't overwrite it.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>