Backport 3.6: Test cases not executed: switch to enforcement mode

[gilles-peskine-arm]

Switch the test coverage analysis in analyze_outcomes.py to enforcement mode. In other words, the CI will now complain if there is a test case that is not executed in any configuration. Legitimate exceptions can be recorded in the script.

This is a backport of https://github.com/Mbed-TLS/mbedtls/pull/9593. Most commits are cherry-picked, but there are a few differences due to how development has already diverged from 3.6.

PR checklist

• [x] changelog not required because: test only
• [x] development PR provided #9593
• [x] framework PR provided Mbed-TLS/mbedtls-framework#58
• [x] 3.6 PR here
• [x] 2.28 PR not required because: too late in the lifetime of 2.28 to bother
• tests provided