Fix #3533: properly escape raw XML object

[vbarbaresi]

Using string formatting with the raw escaped object lead to string evaluation
resulting in "<py._xmlgen.raw object>" in skipped test Junit report

Format the unescaped string first, then use the XML escape method as a last step.

[nicoddemus]

Thanks @vbarbaresi!

[codecov]

Codecov Report

Merging #4167 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #4167      +/-   ##
=========================================
+ Coverage    95.8%   95.8%   +<.01%     
=========================================
  Files         109     109              
  Lines       23828   23836       +8     
  Branches     2352    2352              
=========================================
+ Hits        22829   22837       +8     
  Misses        708     708              
  Partials      291     291

Flag	Coverage Δ
#docs	28.46% <0%> (-0.02%)	⬇️
#doctesting	28.46% <0%> (-0.02%)	⬇️
#linting	28.46% <0%> (-0.02%)	⬇️
#linux	95.66% <100%> (ø)	⬆️
#nobyte	91.92% <100%> (+0.01%)	⬆️
#numpy	93% <100%> (+0.01%)	⬆️
#pexpect	41.41% <0%> (-0.02%)	⬇️
#py27	94.01% <100%> (ø)	⬆️
#py34	92.36% <100%> (+0.03%)	⬆️
#py35	92.37% <100%> (+0.03%)	⬆️
#py36	94.06% <100%> (ø)	⬆️
#py37	92.41% <100%> (+0.03%)	⬆️
#trial	93% <100%> (+0.01%)	⬆️
#windows	94.04% <100%> (ø)	⬆️
#xdist	93.79% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
testing/test_junitxml.py	97.89% <100%> (+0.02%)	⬆️
src/_pytest/junitxml.py	93.33% <100%> (+0.02%)	⬆️

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[coveralls]

Coverage increased (+0.02%) to 94.042% when pulling d72154a on vbarbaresi:3533_junit_escape_skipped into 17655b5 on pytest-dev:master.