Fixed 21157 -- Fixed problems with ResolverMatch

- Fixed bug in `get_callable()` that caused `resolve()` to put a string
  in `ResolverMatch.func`.
- Made `ResolverMatch.url_name` match the actual url name (or `None`).
- Updated tests that used the string value in `ResolverMatch.func`, and
  added regression tests for this bug.
- Corrected test urls whose dummy view paths caused failures (behavior
  that was previously masked by this bug).

See the ticket for more details on the fourth point.