Upload test results to CodeCov

[aik099]

Got an e-mail from CodeCov about a new Test Analytics feature (see https://about.codecov.io/blog/find-failing-and-flaky-tests-with-codecov-test-analytics/ ) and decided to try it out.

Demo: https://app.codecov.io/gh/minkphp/MinkSelenium2Driver/tests/master
To see it in action in this repo please merge this PR first.

P.S.
Copy/paste from minkphp/MinkSelenium2Driver#403 .

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.40%. Comparing base (384bae4) to head (48eb6a5).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #139      +/-   ##
============================================
- Coverage     85.72%   82.40%   -3.33%     
  Complexity      335      335              
============================================
  Files            30       30              
  Lines           932      932              
============================================
- Hits            799      768      -31     
- Misses          133      164      +31     

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