conveniently
(https://github.com/ruby/irb/pull/429)

* Create a base TestIRB::TestCase class

* Save/restore encodings for tests that initializes InputMethod classes

Because `RelineInputMethod#initializes` calls `set_encoding`, which
changes stdio/out/err and Encoding's default encoding values, we need to
make sure any test that directly or indirectly (e.g. through Context)
initializes `RelineInputMethod` restores encodings.

`ReadlineInputMethod` also changes encodings but currently no tests
cover it.

* Remove unnecessary TestHelper module

Since we now have a base TestCase, without_rdoc can just live there.

https://github.com/ruby/irb/commit/c2874ec121