Outcome analysis: detect obsolete exceptions

[gilles-peskine-arm]

In outcome_analysis.py (possibly with an impact on analyze_outcomes.py in consuming repositories), add a way to detect obsolete exceptions to coverage checks.

Note that this is different from the existing mechanism that causes a failure if a test case is expected to be skipped, but actually passes. The goal here is to detect exceptions that do not match any test case, presumably because the corresponding test case has disappeared.

This may be costly, and is of low importance, so it can be a special command line option that we run manually, rather than something done at every invocation.

Example: Mbed-TLS/mbedtls@b62279d removed some exceptions that were no longer necessary because we removed (stopped generating) some test cases that were never executed. The goal of this issue is to have a way to detect those exceptions.