[BUG] The `tests` directory strcutrue is malformed

There is a nested `tests` with `test_brainreg_napari.py` existing outside the nested folder.

This structure should be unwrapped. The directory structure should be:

```
tests
├── conftest.py
├── data
├── test_brainreg_napari.py
├── test_backend
├── test_integration
└── test_unit
```

Instead of:

```
tests
├── conftest.py
├── data
├── test_brainreg_napari.py
└── tests
├── test_backend
├── test_integration
└── test_unit
```

Start by inspecting the repository's tests directory and compare it with the requested layout. Unwrap the nested tests directory so test_backend, test_integration, and test_unit sit directly under tests, then verify that the test suite remains discoverable.

Written by the indexing model from the issue text.