Upgrade minitest to silence warning

The warning in question (printed via `rake test`) was:

    .../lib/ruby/gems/3.4.0/gems/minitest-5.25.1/lib/minitest/mock.rb:33:
      warning: redefining 'object_id' may cause serious problems

Gemfile.lock

@@ -47,7 +47,7 @@ GEM
     logger (1.6.1)
     mini_mime (1.1.5)
     mini_portile2 (2.8.7)
-    minitest (5.25.1)
+    minitest (5.25.4)
     minitest-reporters (1.7.1)
       ansi
       builder