Upgrade minitest to match Rails

Author: aidanharan

Gemfile

@@ -11,7 +11,8 @@ gem "pg", "1.5.9"
 gem "sqlite3", ">= 2.1"
 gem "tzinfo-data", platforms: [:mingw, :mswin, :x64_mingw, :jruby]
 gem "benchmark-ips"
-gem "minitest", ">= 5.15.0"
+gem "minitest", "~> 6.0"
+gem "minitest-mock"
 gem "msgpack", ">= 1.7.0"
 
 if ENV["RAILS_SOURCE"]