Gamemode filepaths break error context logger.

`debug.getinfo` returns the test file's absolute path, which for gamemode files starts with `gamemodes/<gamemode_name>/...`. This path is not valid and should instead be `<gamemode_name>/...`

This is just a specific edgecase for gamemode files, which are not supported by default anyway (without using `GLuaTest_RunTestFiles` to insert the tests manually).