Update wp-browser to v3.5

[lucatume]

Part of Innovation Day work.

This PR updates wp-browser to version 3.5 and, along with it, Codeception to version 4 and PHPUnit to version 9.6.

I've updated the test suite configurations (not really required, but it slims them down) and update the tests to use the correct PHPUnit assertions as required.

There is an hefty number of changed files, but the changes pretty much live all in tests and the test themselves have been updated, not changed in nature.

Note: code style checks are running on the test code, and failing.