selftests/bpf: structure test_{progs, maps, verifier} test runners uniformly

selftests/bpf: structure test_{progs, maps, verifier} test runners uniformly

It's easier to follow the logic if it's structured the same.
There is just slight difference between test_progs/test_maps and
test_verifier. test_verifier's verifier/*.c files are not really compilable
C files (they are more of include headers), so they can't be specified as
explicit dependencies of test_verifier.

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