Improve GitHub reporting of ABI-check

[gilles-peskine-arm]

We sometimes merge pull requests when the ABI-check job fails, because it has common false positives that need to be manually reviewed.

We should change the reporting of ABI-check on GitHub so that:

• If abi_check.py returns 1, the pr-merge job is recorded as a success.
• There is a separate status for ABI-check on GitHub, which is recorded as a failure if abi_check.py returns nonzero or cannot run.

Also, abi_check now does more than just an ABI check: it also checks the API, some systematically generated tests, and some storage tests. So we should give it a clearer name in the new reporting.