Mocke at() matcher is being deprecated with phpunit9 and
will be removed with phpunit10.

Source: https://github.com/sebastianbergmann/phpunit/issues/4297

Luckily we are using those deprecated matchers only in completionlib
tests, so there aren't many cases to modify. Now, we are using
supported matchers (once, exactly, never...) and the tests have
been reorganised to better represent the expected behavior (how
many times stuff is called, with which params and return values).