Pin minitest to < 6 to fix missing minitest/mock

Minitest 6.0 moved mock.rb to a separate minitest-mock gem,
breaking `require "minitest/mock"` in tests. Pin to < 6 to
retain the built-in mock support.

Author: Twill

gems.rb

@@ -15,7 +15,7 @@
 group :test do
   gem "capybara", ">= 3.26"
   gem "mutex_m" # Needed for RubyMine debugging.  Try removing it.
-  gem "minitest", require: false
+  gem "minitest", "< 6", require: false
   gem "minitest-stub-const", require: false
   gem "simplecov", require: false
   gem "rspec", require: false