test-bot: clean up on Jenkins before building PR

Wipe everything from the Homebrew prefix except for the `Library/Taps`
directory to avoid having to refetch `homebrew/core` every time.

Library/Homebrew/cmd/test-bot.rb

@@ -285,6 +285,7 @@ module Homebrew
         @url = ENV["ghprbPullLink"]
         @hash = nil
         test "git", "checkout", "origin/master"
+        test "git", "clean", "-qxdff", "--exclude=/Library/Taps/"
       # Use Travis CI pull-request variables for pull request jobs.
       elsif travis_pr
         @url = "https://github.com/#{ENV["TRAVIS_REPO_SLUG"]}/pull/#{ENV["TRAVIS_PULL_REQUEST"]}"