The dot ssyntax in this case is just what the caller would have to do if the callee expected a pre-testified arg:
(some testify.ch chars)
So the choice of dot instead of colon might be better for consistency - it looks like it's just inlining existing code.
I like it less this way, personally - it looks like "testify" is somehow part of the param name, whereas with "testify:test" the two parts seem more distinct, that "testify" is something you apply to "test" before proceeding with the function. But maybe that's just subjective.