Ability to determine session freshness state

[aik099]

When dealing with shared session strategies it's hard to understand, whether the returned session was just created or was inherited from a previously executed test. This is not a problem now thanks to the $this->getSessionStrategy()->isFreshSession() code, that can be used anywhere during the test execution.

The practical use case for this functionality is to be able to perform login/logout once for all tests in the test case. Examples in the documentation were also updated to reflect this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.58%. Comparing base (f1bb9db) to head (8999227).
Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #128      +/-   ##
============================================
- Coverage     86.16%   82.58%   -3.58%     
  Complexity      330      330              
============================================
  Files            29       30       +1     
  Lines           860      867       +7     
============================================
- Hits            741      716      -25     
- Misses          119      151      +32     

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