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>

kunit: tool: Report an error if any test has no subtests

It's possible for a test to have a subtest header, but zero valid
subtests. We used to error on this if the test plan had no subtests
listed, but it's possible to have subtests without a test plan (indeed,
this is how parameterised tests work).

Tests with 0 subtests now have the result NO_TESTS, and will report an
error (which does not halt test execution, but is printed in a scary red
colour and is noted in the results summary).

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