Add rbtdb setownercase/getownercase unit test

This commit adds a unittest that tests private rdataset_getownercase() and rdataset_setownercase() methods from rbtdb.c. The test setups minimal mock dns_rbtdb_t and dns_rbtdbnode_t data structures.

As the rbtdb methods are generally hidden behind layers and layers, we include the "rbtdb.c" directly from rbtdb_test.c, and thus we can use the private methods and data structures directly. This also opens up opportunity to add more unittest for the rbtdb private functions without going through all the layers.

Closes #2788 (closed)