Make sure we use the API to create a new proxy. The tests depend on that
expectation.
Fixed power tests by disabling the ones for PyProxy that expected
behavior that cannot be created with a pure Python proxy implementation.

Undid one previous checking where the DummyChecker only records the first
checked name.

In several forbidden tests, add a few allowed names, since those are
needed to make the tests pass. In the Python version of the proxy, some
__<name>__ methods are used where the C implementation gets away without
them.