Fix ambiguous argument warning

Fixes:

```
/test/test_spawn.rb:325: warning: ambiguous first argument; put parentheses or a space even after `/' operator
```

Author: eileencodes

test/test_spawn.rb

@@ -322,7 +322,7 @@ def test_spawn_raises_exception_on_unsupported_options
       end
     end
 
-    assert_match /oops/, exception.message
+    assert_match(/oops/, exception.message)
   end
 
   ##