Minor fix to make deprecation tests pass when using rake testW0

[git-p4: depot-paths = "//src/minitest/dev/": change = 14270]

Author: zenspider

test/minitest/metametameta.rb

@@ -27,6 +27,7 @@ def error_on_warn?
   end
 
   def assert_deprecation re = /DEPRECATED/
+    re = // if $-w.nil? # "skip" if running `rake testW0`
     assert_output "", re do
       yield
     end