tests: don't try to rm an unexisting file

This line is necessary if the test fails. If it passes the file won't
exist anymore and shouldn't be removed.

Library/Homebrew/test/test_integration_cmds.rb

@@ -159,7 +159,7 @@ class IntegrationCommandTests < Homebrew::TestCase
     (HOMEBREW_CACHE/"test").write "test"
     assert_match "#{HOMEBREW_CACHE}/test", cmd("cleanup", "--prune=all")
   ensure
-    (HOMEBREW_CACHE/"test").rmtree
+    FileUtils.rm_f HOMEBREW_CACHE/"test"
   end
 
   def test_readall