Compile and run mptcp bpf selftests in CI

[geliangtang]

We need to add mptcp bpf selftests in CI together with other selftests too.

Compile bpf selftests: cd tools/testing/selftests/bpf && make

Run mptcp bpf selftests: ./test_progs -t mptcp

[matttbe]

remaining task: integrate that in GitHub Action → I'm working on that.

[matttbe]

remaining task: integrate that in GitHub Action → I'm working on that.

Done: 71a9e1d223e484148778e2549adbf18a6abecf8a