scripts should exit rather than return

some shells are pickier about this.  darwin uses a newer version of sh
that complains about it.  this is the ‘proper’ behavior in either
scenario.

run_tests.sh

@@ -19,8 +19,8 @@ done
 
 if [ "$failed" = "true" ]; then
   echo "Tests failed!"
-  return 1;
+  exit 1;
 else
   echo "All tests passed!"
-  return 0;
+  exit 0;
 fi