Polyfill new phpunit 9.1 assertions

We should polyfill the new assertions added to phpunit 9.1 in our bridge, so that they can be used on lower versions of php:
https://github.com/sebastianbergmann/phpunit/blob/9.1.5/ChangeLog-9.1.md#910---2020-04-03

See links in https://github.com/symfony/symfony/pull/32922 for inspiration.

See also https://github.com/sebastianbergmann/phpunit/pull/4210/files

Help wanted!
