scripts: compliance: Add a new formatted failure method and class

Format the JUnit failure according to the standard:
https://www.ibm.com/docs/en/developer-for-zos/14.2?topic=formats-junit-xml-format



This also includes a new FmtdFailure class, which will be used in
subsequent commits to implement GitHub-compatible annotations. A list of
FmtdFailure instances is kept in the ComplianceTest instance because
the TestCase instance cannot restore classes derived from Result that
the junitparser module doesn't know about.

Use the new function to format checkpatch errors.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>