Restructure tests

Currently the our test suite is disorganized and slow. This issue will restructure our test suite. We will do the following:

1. We should strictly distinguish unit tests and integration tests
2. Unit tests should be located under test/unit, and the folder structure should follow exactly that of the src/hssm folder. For example, if there is a `src/hssm/distribution_utils/jax.py`, then its unit tests should be located under `tests/distribution_utils/test_jax.py`.
3. Integration tests should be located under `tests/integration`
4. end-to-end tests should be located under `tests/e2e`
5. There should also be performance tests. Those should be located under `tests/performance`
6. slow tests are marked as `slow` and are run separately on CI.

In addition, there are way too many slow tests which makes CI workflows take a very long time. We are going to reevaluate whether these tests are necessary

@cpaniaguam @AlexanderFengler @krishnbera Please feel free to add sub-issues under this issue if you have additional items that need to be worked on

Start by inventorying the existing test suite and comparing it with the src/hssm structure. Use the issue's target locations—tests/unit, tests/integration, tests/e2e, and tests/performance—as the organization checklist, then review slow-test markers and CI handling. Done means tests are separated into the stated categories and slow tests are identified for separate CI runs.

Written by the indexing model from the issue text.