audit: recommend use of assert_match.

It produces significantly easier output to debug than doing `assert` and
`.include?` (which just shows if it passed or failed).

Closes Homebrew/homebrew#41662.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/audit.rb

@@ -794,6 +794,10 @@ class FormulaAuditor
       problem "Use the `#{method}` Ruby method instead of `system #{system}`"
     end
 
+    if line =~ /assert .*\.include?/
+      problem "Use `assert_match` instead of `assert ...include?`"
+    end
+
     if @strict
       if line =~ /system (["'][^"' ]*(?:\s[^"' ]*)+["'])/
         bad_system = $1