tests: add missing require

Amends e4d0187120. The `require` was
accidentally omitted causing the tests to fail very sporadically (or
always, when invoked as `brew tests --only=integration_cmds`).

Library/Homebrew/test/test_integration_cmds.rb

@@ -2,6 +2,7 @@ require "bundler"
 require "testing_env"
 require "fileutils"
 require "pathname"
+require "formula"
 
 class IntegrationCommandTests < Homebrew::TestCase
   def setup