(https://github.com/ruby/irb/pull/464)

* Add debug command tests that don't require yamatanooroti

* Remove debug command related yamatanooroti tests

As discussed in https://github.com/ruby/irb/pull/449#pullrequestreview-1187255149,
we should avoid adding new tests that need yamatanooroti because it's
not maintained by the Ruby org. And since debug commands are now tested
in `test/irb/test_debug_cmd.rb`, we don't need these tests anymore.

* Test against latest debug gem

https://github.com/ruby/irb/commit/78a8aa8834