Obtain failed test results and the associated output from MTR

This task covers identifying and implementing in buildbot a more reliable way of parsing the MTR logs to obtain:

1. A list of failing test cases and their relevant output
2. A list of failed cleanup tests / warnings

Currently, MTR Log Observer uses a regex to identify the relevant pieces of text, a regex that is run line-by-line. This is run in the buildbot event loop, which is single threaded and block processing of other events (such as new logs ingestion, builder commands, etc.)

There are a few possible approaches here:

• Implement a "test collection" step after the actual MTR run.
• Leverage MTR's --xml output.
• Extend MTR to provide a "machine readable" JSON format, that is free of stdout race conditions.

Either, or a combination of the above should allow extracting the right information.

This information must be fed to the crossreference plugin.