Add a test `term.Terminal` implementation for extension authors to use in tests

[BagToad]

It would save extension authors from having to implement their own interface and their own test implementation 🙂

[hernandezadonai587]

Vamos bien

[efegokdemir]

I plan to add a small test terminal implementation in the term package, keeping the production Term API unchanged and covering the test double with focused unit tests.