Selftests are not run against each version of libbpf

We have the file `selftests/.libbpf-versions.txt` which should allow us to run each selftest against each version of libbpf which would be checked out in the git submodule. This doesn't appear to be the case.

Start with selftests/.libbpf-versions.txt and the selftest invocation and git submodule configuration that consume it. Check how the listed libbpf versions are selected, then verify the selftests run for each one. Done means every version listed in the file is checked out and tested.

Written by the indexing model from the issue text.