cleaner: add test to demonstrate that empty prefixes are removed

Library/Homebrew/test/test_cleaner.rb

@@ -11,7 +11,7 @@ class CleanerTests < Test::Unit::TestCase
   end
 
   def teardown
-    @f.prefix.rmtree
+    @f.prefix.rmtree if @f.prefix.exist?
   end
 
   def test_clean_file
@@ -28,6 +28,11 @@ class CleanerTests < Test::Unit::TestCase
     assert_equal 0100444, (@f.lib/'i386.dylib').stat.mode
   end
 
+  def test_prunes_prefix_if_empty
+    Cleaner.new @f
+    assert !@f.prefix.directory?
+  end
+
   def test_prunes_empty_directories
     subdir = @f.bin/'subdir'
     subdir.mkpath