selftests/bpf: add test selectors by number and name to test_progs

Add ability to specify either test number or test name substring to
narrow down a set of test to run.

Usage:
sudo ./test_progs -n 1
sudo ./test_progs -t attach_probe

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>