add loadgroup to test_shutdown_on_sigterm

[mattseddon]

I've seen test_shutdown_on_sigterm fail a few times in the CI:

• https://github.com/iterative/datachain/actions/runs/13183455150/job/36799824090
• https://github.com/iterative/datachain/actions/runs/13185638547/job/36808507612
• https://github.com/iterative/datachain/actions/runs/13185638547/job/36808498643

My assumption is that the following line is causing issues when the tests are distributed between workers (we use tmp_dir fixture).

pathlib.Path("ready").touch(exist_ok=False)

I've added them to the same xdist loadgroup so they won't be able to run into one another.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.74%. Comparing base (86fc806) to head (f2c999e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #902      +/-   ##
==========================================
- Coverage   87.76%   87.74%   -0.02%     
==========================================
  Files         130      130              
  Lines       11639    11639              
  Branches     1579     1579              
==========================================
- Hits        10215    10213       -2     
- Misses       1029     1031       +2     
  Partials      395      395              

Flag	Coverage Δ
datachain	87.67% <ø> (-0.02%)	⬇️

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