test: Remove test-message filenames from generated messages

test: Remove test-message filenames from generated messages

The filenames aren't predictable (including the current directory) nor
stable from one run to the next (including the PID). This makes it
hard to predict the output from a search command that returns such a
message (such as "*").

The original goal was simply to ensure that each generated message was
distinguishable somehow. So just use the message counter instead.