xmlrunner: Expose python docstrings as comments in the XML result file

[oliviermartin]

These comments could for instance be retrieved by XSLT to add
description to test title.

[coveralls]

Coverage increased (+0.004%) to 99.535% when pulling 3fe2e4b on labapart:add-docstring-to-xml into 67bc432 on xmlrunner:master.

[coveralls]

Coverage decreased (-0.1%) to 99.389% when pulling b79520e on labapart:add-docstring-to-xml into f58d7b7 on xmlrunner:master.

[codecov-io]

Codecov Report

Merging #214 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #214   +/-   ##
=======================================
  Coverage   99.52%   99.53%           
=======================================
  Files          14       14           
  Lines        1482     1490    +8     
=======================================
+ Hits         1475     1483    +8     
  Misses          7        7           

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[dnozay]

please add positive / negative testcases; and in particular, please cover cases where __doc__ looks like xml.

[oliviermartin]

please add positive / negative testcases; and in particular, please cover cases where __doc__ looks like xml.

I added the positive/negative testcases.

As far as I can see the build fails but it is due to HEAD of master being broken.