Disable minitest plugins

The latest version of simplecov comes with a minitest plugin in order to
play nice with minitest's `at_exit` hook. However, we don't use
Minitest's `at_exit` hook, so it actually breaks things for us, because
now we don't get `simplecov`'s `at_exit` hooks at all.

To workaround it, I disable minitest plugin loading so that the previous
behavior will be kept when upgrading simplecov.

test/support/test_case.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
+ENV["MT_NO_PLUGINS"] = "yes, no plugins"
 require "minitest"
+
 require "byebug"
 require "byebug/core"
 require "byebug/interfaces/test_interface"