nmtst: improve test macros FAIL() and ASSERT()

nmtst: improve test macros FAIL() and ASSERT()

Fail only printed a message to stderr and exited.
Instead, print it with g_error(), which also breaks
in the debugger and produces a core-dump.

Also, it constructed the message based on an unchecked
format string and constructed a format string dynamically.
Just don't do that.

Also add a comment that these macros are discouraged because
they are cumbersome to write (requiring a test-name and a failure
message).