Update expected atom count in SepTop solvent slow test

[IAlibay]

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[github-actions]

No API break detected ✅

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.39%. Comparing base (1419609) to head (0220bfb).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../tests/protocols/openmm_septop/test_septop_slow.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1598      +/-   ##
==========================================
- Coverage   95.46%   93.39%   -2.08%     
==========================================
  Files         174      174              
  Lines       14584    14584              
==========================================
- Hits        13923    13620     -303     
- Misses        661      964     +303     

Flag	Coverage Δ
fast-tests	93.39% <0.00%> (?)
slow-tests	?

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[hannahbaumann]

Thanks @IAlibay , lgtm!