Refactor generateToc tests to use normal project structure (Qiskit#1267)

This is to unblock Qiskit#1249.
We now use the project folder that `Pkg.mock().outputDir()` would use.

This also removes inline snapshots. They can't be updated via `npm test
-- -u` due to Prettier, so it's more helpful to use a normal
`expect().toEqual()`.