Use non-random cell ids in tests

[mwouts]

This should fix #747

[codecov]

Codecov Report

Merging #751 (397ea69) into master (58e8b27) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #751   +/-   ##
=======================================
  Coverage   99.10%   99.10%           
=======================================
  Files         105      106    +1     
  Lines        9926     9939   +13     
=======================================
+ Hits         9837     9850   +13     
  Misses         89       89           

Impacted Files	Coverage Δ
tests/conftest.py	100.00% <100.00%> (ø)
tests/test_cell_id.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 58e8b27...397ea69. Read the comment docs.