Test fixture root is not correct for some tests

[jaytaph]

When running make test, the system will run ./src/bin/parser-test.rs. This tries to find the tests in FIXTURE_ROOT, but they are not present there. However, the tests in the gosub_html5 crate, do find these tests at the correct position, mainly because the FIXTURE_ROOT is a relative path.

We somehow must make this more consistent so we can run both make test, and have the CI pass