MDL-63097 phpunit: decouple user lastping and session start in tests

MDL-63097 phpunit: decouple user lastping and session start in tests

Previously the test was assuming that the user last ping, when
accessing to a chat should match the first (enter) system message
sent to the chat. And that's ok 99% of times but, if seconds advance
between the creation of the chat_users record and the chat_messages
one, then they don't match.

So now, the assumption is not used anymore and the expectation
is the timestamp of that first (enter) system message, explicitly.