Our API ref says "Valid values are json, xml, or plain. The default is
plain." There's no reason our tests ought to use an invalid value; it
will confuse anyone looking at tests for how to do a thing.
Note that tests were passing because invalid values are ignored, so
?format=txt behaves exactly like ?format=plain.
Change-Id: I6e119cc9c7297d8aade9736fa1d6f4a105466d77
ace2357c62