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

tools/testing/selftests/bpf/prog_tests/module_fentry_shadow.c

@@ -61,6 +61,11 @@ void test_module_fentry_shadow(void)
 	int link_fd[2] = {};
 	__s32 btf_id[2] = {};
 
+	if (!env.has_testmod) {
+		test__skip();
+		return;
+	}
+
 	LIBBPF_OPTS(bpf_prog_load_opts, load_opts,
 		.expected_attach_type = BPF_TRACE_FENTRY,
 	);