test-bot: prevent infinite loop caused by unavailable formula.

Noted, `brew tap foo` will not fail if the tap is already installed.

Library/Homebrew/cmd/test-bot.rb

@@ -247,6 +247,7 @@ module Homebrew
     def safe_formula_canonical_name(formula_name)
       Formulary.factory(formula_name).full_name
     rescue TapFormulaUnavailableError => e
+      raise if e.tap.installed?
       test "brew", "tap", e.tap.name
       retry unless steps.last.failed?
     rescue FormulaUnavailableError, TapFormulaAmbiguityError, TapFormulaWithOldnameAmbiguityError