ABI-check: convey which part failed on GitHub

[gilles-peskine-arm]

The “ABI-check” job can fail for three kinds of reasons:

• API change
• ABI change
• Loss of test coverage

The goal of this test is to make the failure reason(s) apparent in the GitHub reporting. Currently, all we see is “Failures: ABI-API-checking”. Instead we should see something like “Failures: ABI, test coverage”.

This will need changes to the Groovy code but may also need changes to abi_check.py.

We should do this before we do API-breaking changes for 4.0, because we expect to have a lot of API breaks but we don't want do accidentally ignore a loss of test coverage.

Related: #48