Can't find the project's tests if not run from the root

[giacomocavalieri]

It looks like gleeunit is assuming to be run from the project's root. If we do the following it won't find any test:

gleam new foo
cd foo/test    # Or any other subdirectory
gleam test
# 0 tests run...

[lpil]

Thank you. We'll have to ensure this works for both targets.