add test for synthetic stats option

[ix0rai]

the 'include synthetic' option for generating mapping stats is largely undocumented and completely untested. we need to define its expected behaviour and add a unit test to ensure it maintains that behaviour.
tasks:

• do we want the option to include all synthetic entries or just parameters, as is specified in a string in the UI?
  • update javadoc in GenerationParameters to reflect our choice
  • add a unit test to ensure proper behaviour