Allow test paths to be platform agnostic on saving

[cscorley]

Hi

This change fixes test reloading on Windows by using path.join instead of string.replace when trying to determine the current expected test path relative to the workspace and the file being saved.

I have not gotten to test if this change also works as expected on other platforms.

Thanks for maintaining this extension! I hope you're having a good holiday season.

[connorshea]

Thanks!