I19-2 new eiger serial test fixes

[noemifrisina]

Closes #405

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.75%. Comparing base (a769650) to head (4501e48).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nexgen/beamlines/i19_2/eiger.py	0.00%	1 Missing ⚠️
src/nexgen/nxs_write/nxclass_writers.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
+ Coverage   44.75%   45.75%   +1.00%     
==========================================
  Files          53       53              
  Lines        4326     4336      +10     
  Branches      558      559       +1     
==========================================
+ Hits         1936     1984      +48     
+ Misses       2272     2231      -41     
- Partials      118      121       +3     

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