tests: fix warnings to fix target_clones test (-Werror)

Author: concatime

tests/target_clones.c

@@ -1,12 +1,18 @@
 /* This will only be available for GCC with glibc for the foreseeable future */
 
+#include <stdlib.h> /* EXIT_*, exit */
+
  __attribute__((target_clones("default,avx2"))) int f(int x)
 {
     return x + 3;
 }
 
-int main(int argc, char **argv)
+int main(void)
 {
-    int y = f(39);
-    return 0;
-}
+    const int y = f(39);
+    if (y != 42) {
+        exit(EXIT_FAILURE);
+    }
+
+    return EXIT_SUCCESS;
+}