More tests for alex innovation json

Added tests of the JSON function SET_FIELD, plus a few more for FIELD, including with dotted path and/or array index notation, and with dot and bracket characters not used in that way (and therefore escaped with a backslash), and using numeric, floating point data.
As stated elsewhere in this review, there should be test cases for each of the arguments to the JSON functions being null, especially the final argument to SET_FIELD -- and it should probably be tried with path arguments like "field", "array[0]" for a multi-element array and "array[-1]". I'm not 100% sure what output to expect, but we should at least verify the current behavior -- so we can document it.