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 <fallentree@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210817044732.3263066-2-fallentree@fb.com

tools/testing/selftests/bpf/test_progs.c

@@ -755,7 +755,7 @@ int main(int argc, char **argv)
 	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;
 	}
@@ -803,7 +803,7 @@ int main(int argc, char **argv)
 		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();