Remove lib/resque/core_ext/kernel.rb and instead put it in test_helper.rb

As discussed in https://github.com/resque/resque/commit/df62b058030ba95f86761bb9ab7bf34e63db0a46#commitcomment-2995830

I don't like the duplication that results from this, OTOH

> you'll find a bunch of tests in test/legacy. These are the older Resque tests. Don't look at them unless your code breaks them. Consider these black-box acceptance tests. We don't like them, but we want to make sure we're not breaking anything.

so maybe removing a shared dependency is actually a good thing.