Rewrite messages of unit test

[rugk]

Currently all unit test messages (the ones in "assert…") describe the case when the test/assert passes/is satisfied. However, as they are only printed when the assert actually fails, they should better describe the opposite case: What actually fails.

This is an easy wording task. Just turn around the meaning of all these messages. (Remember to use present tense.)

Example: "Should print 1234."
Change to: "Does not print 1234."