33601 add json helper tests

Created tests for JSONHelper, only one test per method, since this class
is extensively tests via other tests which test class which encode and
decode json data.

Main Question:
Are these tests enough?


This for loop duplicates the one above. I'm almost certain you don't need to convert from a TreeMap to a HashMap. Even though the Map implementations are different, equals() will return true if they have the same contents.