MDL-65093 core_message: remove some "dupe" tests
With the deprecation of can_post_message() in favor of
can_send_message() all the new test_can_send_message_xxx()
were created from the original test_can_post_message_xxx() ones.
So, really we don't need all the old ones. Just ensure that
debugging is being called and done. New tests are already covering
the new method.
Note this is not strictly needed but in practice is allowing
MySQL + PHP 7.3 to stop segfaulting (because of some sort of limit /
overflow happening somewhere else). So this is just a practical
fix to avoid that while all the messaging tests are reorganizaed
@ MDL-63670. There are some good things to fix there.
