feat(ngMock): decorator that adds Scope#$countChildScopes and Scope#$countWatchers

When writing tests it's often useful to check the number of child scopes
or watchers within the current current scope subtree. Common use-case for advanced
directives is to test that the directive is properly cleaning up after itself. These
new methods simplify writing assertions that verify that child scopes were properly
destroyed or that watchers were deregistered.

Rehash of #9871 to see if it can land.