tests: add seed to lc2st session fixtures

[janfb]

CD was failing with a flaky LC2ST test. The reason is that even though we have function-scoped always true seeding, this will not be effective for session-scoped fixtures. The session-scoped fixtures will be executed at module loading and are therefore subject to RNG state changes, e.g., due to dependency changes.

That's why the seemingly unrelated NPE-PFN merge now led to this flaky test fail. The reason why this failed now was likely a different order or versions of underlying packages.

An additional seeding in the fixtures fixes this problem.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.81%. Comparing base (839762d) to head (3beda11).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1864      +/-   ##
==========================================
+ Coverage   87.76%   87.81%   +0.04%     
==========================================
  Files         143      143              
  Lines       13317    13402      +85     
==========================================
+ Hits        11688    11769      +81     
- Misses       1629     1633       +4     

Flag	Coverage Δ
fast	81.30% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.
see 1 file with indirect coverage changes

[dgedon]

looks good. Maybe you can briefly explain for PR documentation what in NPE-PFN merger caused this. Just for future reference