Switch to testing::fixture (#4211)
### Description
The old `test_generator::test_resources` could only find resources
relative to the project root, and not relative to the test file. Because
of the new nextpack directory structure, this causes all kinds of
headaches between running in nextpack and running in turbo
(`crates/turbopack-tests` vs
`nextpack/turbo-crates/crates/turbopack/tests`).
We'll need to do the same in `next-dev-tests` in Next.js repo.
### Testing Instructions
```bash
$ cd turbo
$ cargo run nextest -p turbopack-tests
$ cd nextpack
$ node ./scripts/init.cjs
$ cargo run nextest -p turbopack-tests
```
