Added some extra CUnit integration test coverage
- One test is disabled due to a bug
- Other new tests demonstrate some insufficiencies in design
Allow cunit sources to #include "gradle_cunit_register.h"
Don't use ModelRules in CUnit plugin since they don't play well with sources block (yet)
- Split rule-based NativeBinariesTestModelPlugin from standard NativeBinariesTestPlugin
Put generated CUnit launcher sources into a separate source set