kunit: kunit_tool: Correctly parse diagnostic messages

Currently, kunit_tool expects all diagnostic lines in test results to
contain ": " somewhere, as both the subtest header and the crash report
do. Fix this to accept any line starting with (minus indent) "# " as
being a valid diagnostic line.

This matches what the TAP spec[1] and the draft KTAP spec[2] are
expecting.

[1]: http://testanything.org/tap-specification.html
[2]: https://lore.kernel.org/linux-kselftest/CY4PR13MB1175B804E31E502221BC8163FD830@CY4PR13MB1175.namprd13.prod.outlook.com/T/

Signed-off-by: David Gow <davidgow@google.com>
Acked-by: Marco Elver <elver@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>