Assume some warnings might not have the full PROJECT_ROOT

In the case of minitest assert_nil:

> Use assert_nil if expecting nil from test/application/configuration_test.rb:4031

We can cheat and expect any warning that has a path which includes
"test/*.rb" should exist before raising in CI.

If the message doesn't contain a path like that, and is not included in
the PROJECT_ROOT then we can ignore it.

Author: zzak

tools/strict_warnings.rb

@@ -30,7 +30,8 @@ def warn(message, ...)
 
     super
 
-    return unless message.include?(PROJECT_ROOT)
+    testpath = message[/test\/.*\.rb/]&.chomp || message
+    return unless message.include?(PROJECT_ROOT) || Pathname.new(testpath).exist?
     return if ALLOWED_WARNINGS.match?(message)
     return unless ENV["RAILS_STRICT_WARNINGS"] || ENV["BUILDKITE"]