Add unittest2 testing to trial environment

[nicoddemus]

Just noticed that test_usefixtures_marker_on_unittest is parametrized
for unittest2, but no environment ever installed that library.

[codecov]

Codecov Report

Merging #4823 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4823      +/-   ##
==========================================
+ Coverage   95.64%   95.66%   +0.01%     
==========================================
  Files         113      113              
  Lines       25163    25163              
  Branches     2498     2498              
==========================================
+ Hits        24066    24071       +5     
+ Misses        773      772       -1     
+ Partials      324      320       -4

Flag	Coverage Δ
#linux	95.45% <ø> (ø)	⬆️
#osx	93.3% <ø> (+0.01%)	⬆️
#windows	93.67% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/_pytest/capture.py	94.11% <0%> (+0.45%)	⬆️
src/_pytest/cacheprovider.py	97.16% <0%> (+1.41%)	⬆️

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