Replace at feature in SessionManagerTest unit test

[rdeutz]

Pull Request for Issue #42528 .

Summary of Changes

Remove the at() function because it is deprecated for phpunit10.

Site note: Can be discussed if the test has make sense in the past, here I only made it run.

Testing Instructions

run the unit test e.g. phpunit --testdox --filter SessionManagerTest

Actual result BEFORE applying this Pull Request

deprecated messages for the test

Expected result AFTER applying this Pull Request

no deprecated messages for the test

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[brianteeman]

This does not resolve the reported issue. "phpunit it out of date and unsupported"

[rdeutz]

I want to do it in two steps, one to fix the test and when this is done update the phpunit version

[brianteeman]

thats perfect and expected but you shoujldnt have closed the issue until both are done

[wilsonge]

Thanks!