Move `fixture`s into `conftest.py`

[paddyroddy]

Fixes #288

[deprecated-napari-hub-preview-bot[bot]]

Preview page for your plugin is ready here: https://preview.napari-hub.org/quantumjot/btrack/331 Updated: 2023-08-07T15:47:16.128172

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (44a1187) 86.00% compared to head (86482ed) 86.00%.

:exclamation: Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #331 +/- ## ======================================= Coverage 86.00% 86.00% ======================================= Files 30 30 Lines 2165 2165 Branches 393 393 ======================================= Hits 1862 1862 Misses 223 223 Partials 80 80 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.