audit: check for HOMEBREW_PREFIX test path usage.

This is also not allowed.

As seen in https://github.com/Homebrew/homebrew-core/pull/6996.

Library/Homebrew/dev-cmd/audit.rb

@@ -760,7 +760,7 @@ class FormulaAuditor
     end
     bin_names.each do |name|
       ["system", "shell_output", "pipe_output"].each do |cmd|
-        if text =~ /(def test|test do).*#{cmd}[\(\s]+['"]#{Regexp.escape name}[\s'"]/m
+        if text =~ %r{(def test|test do).*(#{Regexp.escape HOMEBREW_PREFIX}/bin/)?#{cmd}[\(\s]+['"]#{Regexp.escape name}[\s'"]}m
           problem %Q(fully scope test #{cmd} calls e.g. #{cmd} "\#{bin}/#{name}")
         end
       end