test-bot: don't run brew tests etc. when formulae.

If there's formulae changes or the specified argument is a formula then
don't waste time running e.g. `brew tests`.

Library/Homebrew/dev-cmd/test-bot.rb

@@ -649,7 +649,7 @@ module Homebrew
       @category = __method__
       return if @skip_homebrew
 
-      if @tap.nil?
+      if @tap.nil? && Array(@formulae).empty?
         tests_args = ["--official-cmd-taps"]
         tests_args_no_compat = []
         tests_args_no_compat << "--coverage" if ARGV.include?("--coverage")