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 98.02%. Comparing base (16d5347) to head (a51e1eb).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #181   +/-   ##
=========================================
  Coverage     98.02%   98.02%           
  Complexity      127      127           
=========================================
  Files             1        1           
  Lines           304      304           
=========================================
  Hits            298      298           
  Misses            6        6           

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