Rewrite one :discard on queue overflow test for robustness

How about this as a different approach to https://github.com/ruby-concurrency/concurrent-ruby/commit/6af131618813cd882a5d7d3b0d3f2ff7d04a3d6f ? The basic idea is that you take a lock, then inject enough tasks waiting on that lock to fill up all the threads and the queue. Then you inject more tasks which should never run, unlock the lock, and shutdown the executor so that all threads run to completion and your results are predictable.

It passes UT for me, and I *think* this is a robust approach, but the proof of the pudding will be in the eating. If you agree witth the approach, I can definitely reuse it for the other `:discard` tests, and maybe some others.