Clarify usage of test messages

[ts5746]

Original internal issue. The description in par. 4.3.5.2 of the specification on how to use the test message or what they really mean is unclear. Do test messages require a test A(0) before they can be used? Also referring to par. 5.5: if test messages are only allowed on testing networks, then there seems no need for a separate test message anyway. Messages on testing networks are all testing messages, and messages on production networks are all for real.