kunit: tool: fix parsing of test attributes

Add parsing of attributes as diagnostic data. Fixes issue with test plan
being parsed incorrectly as diagnostic data when located after
suite-level attributes.

Note that if there does not exist a test plan line, the diagnostic lines
between the suite header and the first result will be saved in the suite
log rather than the first test case log.

Signed-off-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>