test_bucket: don't try to clean a non-existent keg

It is a no-op under Ruby 1.8 but Pathname#find raises ENOENT if the
directory doesn't exist under 1.9. But we really shouldn't try to clean
a non-existent keg anyway, and FormulaInstaller will have bailed out
long before the clean step if the keg doesn't exist, so lets just not
even test this condition; the cleaner has its own tests anyway.

Alternatively we can adjust the cleaner to handle this case, but I don't
see much value in that.

n.b. with this, the tests now pass under 1.9.3 (for me), which is nice
because I often run them on that version inadvertently.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/test/test_bucket.rb

@@ -84,7 +84,6 @@ class BeerTasting < Test::Unit::TestCase
       assert_nothing_raised do
         f=TestBallWithRealPath.new
         Homebrew.info_formula f
-        Cleaner.new f
         Homebrew.prune
         #TODO test diy function too
       end