Make the integration test system crossplatform

[ExceptionallyHandsome]

Related to #68 and #69

Since the project goes cross-platform, having the tests runnable and working on all platforms - not just linux - becomes a must have. It doesn't have to be completely platform-agnostic, but it must work on Linux, Windows, and macOS.

Docker-based testing will have to be abandoned as well, because there's a big difference between "works in Docker for Windows" and "works on bare Windows", not sure what the situation with macOS is.