Improve log path presentation to the user on Windows

[MisterDA]

When a build fails 😞 alcotest shows a message to tell the user where to find the log file:

Logs saved to `~\Tarides\obuilder\_build\default\test\_build\_tests\MJ8871ZU\manifest.000.output'.

The tilde ~ as an expansion for the home path is only understood by Unix shells. The Unix shells themselves don't understand the backslashes! That path is a bit cumbersome to copy-paste somewhere to cat the log.

I suggest using forward slashes only (Windows understands them (in most cases)), and if possible, show the path relative to the root of the project instead of the home directory, or something that wouldn't involve ~.