More tests on cell markers

[mwouts]

More tests to answer #855

[codecov]

Codecov Report

Merging #856 (915deef) into master (5e96f4d) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #856      +/-   ##
==========================================
+ Coverage   99.07%   99.13%   +0.06%     
==========================================
  Files         109      111       +2     
  Lines       10480    11106     +626     
==========================================
+ Hits        10383    11010     +627     
+ Misses         97       96       -1     

Impacted Files	Coverage Δ
tests/test_cell_markers.py	100.00% <100.00%> (ø)
tests/test_cli.py	100.00% <0.00%> (ø)
tests/test_black.py	100.00% <0.00%> (ø)
tests/test_raw_strings.py	100.00% <0.00%> (ø)
jupytext/cell_to_text.py	97.86% <0.00%> (+0.04%)	⬆️
jupytext/cell_reader.py	97.78% <0.00%> (+1.19%)	⬆️

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