Make tests use local version of this package

Notice that the tests in `tests` using a pinned version of this package, e.g. https://github.com/exercism/gleam-test-runner/blob/52841faf69669bebd90c948a4b79398c36a082d4/tests/all_fail/gleam.toml and 356d594.

The effect is that, when a PR is made in this repo, the CI tests run with the pinned version, *not* the code in the PR. This seems problematic. Further, the tests in `runner` are not run in CI I think.

If, as I suspect, this is not desired behavior, I think three fixes are needed.

1. Make `gleam.toml` in `tests/**` use the local `exercism_test_runner` package by specifying the relative path.
2. Making the [test-in-docker.sh](https://github.com/exercism/gleam-test-runner/blob/52841faf69669bebd90c948a4b79398c36a082d4/bin/run-in-docker.sh) script mount directories such that the local `exercism_test_runner` is available in docker.
3. Adding the `runner` tests to the GitHub Actions CI workflows.

If somebody confirms the above I'm happy to work on it.