Fix CakePHP 5.3 deprecation warnings in tests

[dereuromark]

Summary

• Replace _execute() with process() in test form anonymous class (deprecated since CakePHP 5.3.0)
• Replace $table->searchManager() with $table->getBehavior('Search')->searchManager() in test table classes and behavior tests (calling behavior methods via table proxy deprecated since CakePHP 5.3.0)
• Replace $table->isSearch() with $table->getBehavior('Search')->isSearch() in behavior tests

These 3 deprecation warnings cause the MySQL 8.2 CI job to fail with --display-deprecations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.35%. Comparing base (7132608) to head (ddc132b).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #372   +/-   ##
=========================================
  Coverage     91.35%   91.35%           
  Complexity      240      240           
=========================================
  Files            19       19           
  Lines           694      694           
=========================================
  Hits            634      634           
  Misses           60       60           

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