Skip the tests for jupytext --execute if a timeout occurs

[mwouts]

Closes #489

[codecov]

Codecov Report

Merging #490 into master will decrease coverage by 0.06%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
- Coverage   98.81%   98.75%   -0.07%     
==========================================
  Files          81       81              
  Lines        8050     8060      +10     
==========================================
+ Hits         7955     7960       +5     
- Misses         95      100       +5     

Impacted Files	Coverage Δ
tests/test_execute.py	93.50% <50.00%> (-6.50%)	⬇️

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