switch to use blackbox testing, move tests to separate module

[thaJeztah]

This allows us to remove the test-dependencies from the module itself.

• depends on drop EOL go versions, update golang-ci-lint, gha #33
• depends on testing: use t.Cleanup() for test utilities, remove gotest.tools dependency #21
• depends on go.mod: github.com/creack/pty v1.1.18, golang.org/x/sys v0.1.0 #34

[thaJeztah]

Okay, moved this out of draft for consideration.

[thaJeztah]

Ah, I think this was the bit I was still looking at. It's somewhat unclear why x and y aren't exported; effectively WinSize in this module is identical to golang.org/x/sys/unix.Winsize`;

Type in this module:

// Winsize represents the size of the terminal window.typeWinsizestruct {
Heightuint16Widthuint16xuint16yuint16
}

And in golang.org/x/sys:

typeWinsizestruct {
Rowuint16Coluint16Xpixeluint16Ypixeluint16
}

In this module, we're converting the types between, so perhaps instead we should make it an alias (and deprecate the local type). Let me look at that.

[thaJeztah]

Oh! I recall now; for Windows, it's a different type, so we can make it an alias (but only on !windows)

[thaJeztah]

Hm.. nevermind; field names are different 🤦