test-bot: don't brew tests for formula PRs. (#294)

This adds >1m to every build on every node and it can't actually be
affected/fixed by formula PRs.

Library/Homebrew/cmd/test-bot.rb

@@ -633,11 +633,11 @@ module Homebrew
       return if @skip_homebrew
 
       ruby_two = RUBY_VERSION.split(".").first.to_i >= 2
+
+      if @tap.nil?
         tests_args = []
         tests_args << "--coverage" if ruby_two && ENV["TRAVIS"]
         test "brew", "tests", *tests_args
-
-      if @tap.nil?
         test "brew", "tests", "--no-compat"
       elsif @tap.core_tap?
         readall_args = []