Get test count from collection_finish hook over runtestloop

[nak]

Using runtestloop hook to count tests can easily conflict with other plugins that override the default runtestloop behavior (e.g. in pytest_mproc or pytest_xdist that do distributed testing). Using the collection hooks -- in this case collect_finish hook seems more approriate overall anyway(?)

[codecov-commenter]

Codecov Report

Merging #195 into master will increase coverage by 0.20%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage   87.44%   87.65%   +0.20%     
==========================================
  Files           2        2              
  Lines         478      478              
  Branches       85       85              
==========================================
+ Hits          418      419       +1     
  Misses         35       35              
+ Partials       25       24       -1     

Impacted Files	Coverage Δ
pytest_sugar.py	82.99% <100.00%> (+0.29%)	:arrow_up:

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[Teemu]

Rebased this https://github.com/Teemu/pytest-sugar/pull/253