v2.2.7: Fix re-rendering when prompt is exactly the width of the terminal (#321)

* Fix re-rendering when prompt is exactly the width of the terminal

The code that measures width of rendered lines of content and whether
any have overflown in the terminal did not account for the possibility
that the content could be _exactly_ the width of the terminal. In that
case, it shouldn't be counted as overflow.

* Reduce test dependencies by going straight to `pty`

This skips the go-expect & vt10x dependencies when stubbing only
terminal size and goes straight to the `creack/pty` library, which is
already used by go-expect under its old name, `kr/pty`.

* Use old name of `creack/pty` since it was already vendored