Improves test message prefix.

It now links to the build in the current repo, even if that's not the default.

It also uses ChatExchange@localhost, linked to the git repo, when not
being run on Travis. (I wasn't sure what label to use here.)

Previous, it would generate an invalid or weird Travis link in these cases.
Fixes bug in formatting syntax for Travis test message.
Parameterizes test_se_message_echo over an editable list of TEST_ROOMs.

On the main branches, this list can remain as is so it works with
Manishbot. On my test branch, I'd also like to add Stack Overflow's
sandbox, and later MSO's too, so I can test that it's working on all
hosts.
merge conflict