I have considered different constructs like "where:" to be placed in code blocks (such as function) when working on Next Generation Shell. I have decided against because these constructs will not be executed at runtime (when the function is called) and therefore I assume it will be at least somewhat confusing.

For tests, NGS has "TEST" keyword which can be placed at beginning of any line. The test is everything till end of that line. I typically place these below functions that they test.