Cross compile tests

[Bluebugs]

Is your feature request related to a problem? Please describe:

Currently if we want to run go test on a specific target with fyne, we can't use fyne-cross and have to setup a full build system for that target.

Is it possible to construct a solution with the existing API?

We can not at the moment build go test for any of the OS supported by fyne-cross, but using fyne-cross-image it should be possible to run go test -c to generate the tests binary without running it.

Describe the solution you'd like to see:

Add a -tests flag or equivalent for all OS that will generate the binary that contain all tests, but not run it.