selftests/bpf: Do not unload bpf_testmod in load_bpf_testmod

Do not unload bpf_testmod in load_bpf_testmod, instead call
unload_bpf_testmod separatelly.

This way we will be able use un/load_bpf_testmod functions
in other tests that un/load bpf_testmod module.

Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230515133756.1658301-6-jolsa@kernel.org


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