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

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

[ Upstream commit 971f7c32147f2d0953a815a109b22b8ed45949d4 ]

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

Fixes: aa3d65de4b900 ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>