tests: online::clone: use URL of test server

All our tests running against a local SSH server usually read the
server's URL from environment variables. But online::clone::ssh_cert
test fails to do so and instead always connects to
"ssh://localhost/foo". This assumption breaks whenever the SSH server is
not running on the standard port, e.g. when it is running as a user.

Fix the issue by using the URL provided by the environment.

(cherry picked from commit c2c95ad0)

tests/online/clone.c

@@ -577,7 +577,7 @@ void test_online_clone__ssh_cert(void)
 	if (!_remote_ssh_fingerprint)
 		cl_skip();
 
-	cl_git_fail_with(GIT_EUSER, git_clone(&g_repo, "ssh://localhost/foo", "./foo", &g_options));
+	cl_git_fail_with(GIT_EUSER, git_clone(&g_repo, _remote_url, "./foo", &g_options));
 }
 
 static char *read_key_file(const char *path)