Include `FileUtils` in `spec_helper`.

Library/Homebrew/test/spec_helper.rb

@@ -44,6 +44,8 @@ RSpec.configure do |config|
 
   config.filter_run_when_matching :focus
 
+  config.include(FileUtils)
+
   config.include(RuboCop::RSpec::ExpectOffense)
 
   config.include(Test::Helper::Fixtures)

Library/Homebrew/test/tap_spec.rb

 describe Tap do
-  include FileUtils
-
   alias_matcher :have_formula_file, :be_formula_file
   alias_matcher :have_custom_remote, :be_custom_remote
 
@@ -307,8 +305,6 @@ describe Tap do
 end
 
 describe CoreTap do
-  include FileUtils
-
   specify "attributes" do
     expect(subject.user).to eq("Homebrew")
     expect(subject.repo).to eq("core")