Use Redis for session storage in CI

[dennisameling]

We've had some issues with PHP sessions in the past. Check if using Redis also works, hopefully it will speed up the tests as well!

More context in mautic/mautic#2546 (comment) and https://ma.ttias.be/php-session-locking-prevent-sessions-blocking-in-requests/

[codecov-commenter]

Codecov Report

Merging #252 (9cbc633) into main (0714245) will not change coverage.
The diff coverage is n/a.

❗ Current head 9cbc633 differs from pull request most recent head ff64266. Consider uploading reports for the commit ff64266 to get more accurate results

@@            Coverage Diff            @@
##               main     #252   +/-   ##
=========================================
  Coverage     51.45%   51.45%           
  Complexity      406      406           
=========================================
  Files            30       30           
  Lines          1028     1028           
=========================================
  Hits            529      529           
  Misses          499      499           

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[dennisameling]

Tests are a little faster at 8m25, and we're not dependent on file session storage anymore!

[RCheesley]

Awesome - thanks for looking into this!