Improved : Rewrites FlexibleMapAccessorTest (OFBIZ-13345)

- Rewrites the FlexibleMapAccessorTest in groovy
- Explodes the previous big test in several test cases, a cases for each
feature of the FMA.
- Removes some of the testing lines that were not relevant :
  - Testing that a FMA in a set is correctly contained
  - Testing a FSE instance in this class. FSE has its own test cases.
- Some of the lines were repeated, only with the two compared objects
inverted.
- Creates a dedicated class for the utility test map.

Thanks Jacques for the testing.

Co-authored-by: cgaetan <gaetan.chaboussie@nereide.fr>