selftest/bpf: centralize libbpf logging management for test_progs

Make test_progs test runner own libbpf logging. Also introduce two
levels of verbosity: -v and -vv. First one will be used in subsequent
patches to enable test log output always. Second one increases verbosity
level of libbpf logging further to include debug output as well.

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