samples/bpf: refactor BPF functionality testing scripts

samples/bpf: refactor BPF functionality testing scripts

Currently, some test scripts are experiencing minor errors related to
executing tests.

    $ sudo ./test_cgrp2_sock.sh
    ./test_cgrp2_sock.sh: 22: test_cgrp2_sock: not found

This problem occurs because the path to the execution target is not
properly specified. Therefore, this commit solves this problem by
specifying a relative path to its executables. This commit also makes
a concise refactoring of hard-coded BPF program names.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Link: https://lore.kernel.org/r/20230115071613.125791-3-danieltimlee@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>