selftests/bpf: Load bpf_testmod for verifier test

Loading bpf_testmod kernel module for verifier test. We will
move all the tests kfuncs into bpf_testmod in following change.

Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230515133756.1658301-8-jolsa@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>