add test data

test/testcases/gcd.c

@@ -0,0 +1,15 @@
+#include "io.inc"
+
+int gcd(int x, int y) {
+  if (x % y == 0)
+    return y;
+  else
+    return gcd(y, x % y);
+}
+
+int main() {
+  printInt(gcd(10, 1));
+  printInt(gcd(34986, 3087));
+  printInt(gcd(2907, 1539));
+  return judgeResult % Mod;  // 178
+}