define global shutup method (temporary)

solves the following error:
```bash
$ bundle exec rake test
...
NoMethodError: undefined method `shutup'
...
```

This is not a permanent solution.  The test suite should be changed
around to find the new `shutup` method, which is still in Homebrew,
but no longer global.

test/test_helper.rb

@@ -22,6 +22,24 @@
 # require homebrew testing env
 require 'test/testing_env'
 
+# todo temporary, copied from old Homebrew, this method is now moved inside a class
+def shutup
+  if ARGV.verbose?
+    yield
+  else
+    begin
+      tmperr = $stderr.clone
+      tmpout = $stdout.clone
+      $stderr.reopen '/dev/null', 'w'
+      $stdout.reopen '/dev/null', 'w'
+      yield
+    ensure
+      $stderr.reopen tmperr
+      $stdout.reopen tmpout
+    end
+  end
+end
+
 # making homebrew's cache dir allows us to actually download casks in tests
 HOMEBREW_CACHE.mkpath
 HOMEBREW_CACHE.join('Casks').mkpath