Merge tests/ into fewer test crates

1. Can we speed up linking in CI?
2. If this is not possible, consider merging tests/ files into one or more test crates - as long as this doesn't slowdown the build of a particular test too much. See for more info: https://matklad.github.io/2021/02/27/delete-cargo-integration-tests.html
  Possible remaining crate split:
   - General
   - Numerical
   - Parallel/Rayon