Forgot main_error.cpp in tests.

tests/case/main_error.cpp

@@ -0,0 +1,7 @@
+#include <iostream>
+
+int main(int argc, char *argv[]) {
+  std::cout << "Hello, World!" << std::endl;
+  std::cout << undeclared_variable << std::endl;
+  return 0;
+}