Can't run tests unless Gemfile is in project root

[Tabby]

If the Gemfile for the test suite to be run is not in the project root, the extension fails to find it and thus can't run any tests

We need to:

• Add code to search for Gemfiles that are in a parent dir of the tests to be run, if it's not in the project root
• Pass the relative path to the Gemfile to the subprocess handling code so that it can run in the working directory where the Gemfile is, but reports the correct URIs for tests relative to the workspace root

[Tabby]

I've half-done this. The Gemfile detection code is done and seems to be working, and I will push it to a branch soon. Still need to do the second bit