MDL-45565 core_message: Fixed strangers array

Strangers get mixed with noreply user and support user,
if  noreply/support users real user.
To avoid duplication use userid as array index. Also,
avoid fetching data again for noreply and support users,
if they are real users, then data is already feteched.
MDL-45565 core_message: Added unit tests.
MDL-45565 core_message: Reset internal users in phpunit
MDL-45565 phpunit: Changed testsuite_name, so as to avoid class collision
Merge branch 'wip-mdl-45565' of https://github.com/rajeshtaneja/moodle