[ci] Collect and report segfaults in CI

[driazati]

Right now segfaults usually crash pytest and don't get included in JUnits (see #10681) (however if running under pytest-xdist then only the worker crashes and it is processed like a normal test failure, but running xdist with -n 1 everywhere adds some runtime overhead, though this might be worth investigating more). We should:

1. Fix tlcpack/ci packer builds to correctly set kernel.core_pattern and ulimit -c to collect core dumps (fixed by Fix core_pattern and ulimit settings tlc-pack/ci#29)
2. Look for these at the end of CI runs in pytest_wrapper.py
3. If they are found, run them through gdb to get a stack trace and print it out in the CI logs
4. Include a message about the segfault in the summary

cc @Mousius @areusch @gigiblender