selftests/bpf: Skip module_fentry_shadow test when bpf_testmod is not available

This test relies on bpf_testmod, so skip it if the module is not available.

Fixes: aa3d65de ("bpf/selftests: Test fentry attachment to shadowed functions")
Signed-off-by: Artem Savkov <asavkov@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230914124928.340701-1-asavkov@redhat.com