kunit: tool: Fix spelling of "diagnostic" in kunit_parser

Various helper functions were misspelling "diagnostic" in their names.
It finally got annoying, so fix it.

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