Add tests

[jwodder]

This project needs tests. Possible ways to implement these tests include:

• Use an expect-style library (e.g., rexpect or expectrl) to create & interact with a test terminal in which rustyline-async is doing its thing. This may require implementing some test binary that wraps rustyline-async for invocation by the expect library.

• Use vt100 or similar to calculate what the screen looks like while using rustyline-async and assert that it looks correct.

• Use vhs to record interactions with some test binary that wraps rustyline-async and then assert that the display remains the same after the code changes. ratatui uses this approach for testing its examples.