test_stasis_endpoints.c: Make channel_messages more stable

test_stasis_endpoints.c: Make channel_messages more stable

The channel_messages test was assuming that stasis would return
messages in a specific order.  This is an incorrect assumption as
message ordering was never guaranteed.  This was causing the test
to fail occasionally.  We now test all the messages for the
required message types instead of testing one by one.

Resolves: #158