Add a way to simulate the terminal resizing to Pilot

[willmcgugan]

We could use a way of simulating the terminal sizing in tests.

The API should be something like the following:

class Pilot:
    async def resize_terminal(self, width, height):
        ...

This should send the appropriate resize events to the App.

[github-actions]

Don't forget to star the repository!

Follow @textualizeio for Textual updates.