[#250] Make -cache work with relative path.

[williambrode]

### Description:

Fixes #(issue)

Checklist:

• [ ] Tests included.
• [ ] Lint and formatter run with no errors.
• [ ] Tests all pass.

Where applicable:

• [ ] Public APIs match existing style.
• [ ] Any breaking changes have a deprecation path or have been discussed.
• [ ] Updated the vendor folder (using go mod vendor).

[williambrode]

Perhaps a maintainer could suggest where to add a test for this? I could add it around the immediate function (Mount) but then I'd just be testing the specific implementation.

[Bluebugs]

We currently have mostly unit test in the form of things that test how a function will provide a result depending on its input. That would be something you could easily add as a new file in internal/volume.

But these are not super useful as it only prove that things match your expectation, not that it work.

We do have some better/more useful end 2 end kind of tests with the CI that build full application using fyne-cross, but they are too broad and we don't get any code coverage out of them. I would love to see some more integration tests that actually show fyne-cross work and provide proper tests coverage. I think that is really further away at the moment.