selftests/bpf: Merge test_stub.c into testing_helpers.c

Move testing prog and object load wrappers (bpf_prog_test_load and
bpf_test_load_program) into testing_helpers.{c,h} and get rid of
otherwise useless test_stub.c. Make testing_helpers.c available to
non-test_progs binaries as well.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/bpf/20211103220845.2676888-11-andrii@kernel.org