test(e2ee): Improve e2ee test suite

[artonge]

• Use saveState() and resetState() from @nextcloud/cypress
• Update @nextcloud/cypress to v1.0.0-beta.15 to benefit from clearing sessions on resetState()
• Enable test isolation
• Improve utility functions to broader their uses in tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 2.28%. Comparing base (fff8f64) to head (b73caa7).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #3002      +/-   ##
=========================================
- Coverage    2.28%   2.28%   -0.01%     
=========================================
  Files          97      97              
  Lines        6214    6217       +3     
  Branches      111     111              
=========================================
  Hits          142     142              
- Misses       6072    6075       +3     

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