selftests/bpf: Skip loading bpf_testmod when using -l to list tests.

When using "-l", test_progs often is executed as non-root user,
load_bpf_testmod() will fail and output errors. This patch skips loading bpf
testmod when "-l" is specified, making output cleaner.

Signed-off-by: Yucong Sun <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/test_progs.c

···
 	save_netns();
 	stdio_hijack();
 	env.has_testmod = true;
-	if (load_bpf_testmod()) {
+	if (!env.list_test_names && load_bpf_testmod()) {
 		fprintf(env.stderr, "WARNING! Selftests relying on bpf_testmod.ko will be skipped.\n");
 		env.has_testmod = false;
 	}
···
 		if (test->need_cgroup_cleanup)
 			cleanup_cgroup_environment();
 	}
-	if (env.has_testmod)
+	if (!env.list_test_names && env.has_testmod)
 		unload_bpf_testmod();
 	stdio_restore();