Points hooks: introduce a method to get the current instance number
Points hooks: unregister an instance when it is deleted
AJAX points test case: unregister any stray hook instances on setUp
Unit tests functions: allow more than one instance of a hook to be created

Updates the `wordpointstests_add_points_hook()` function to be able to
create more than one points hook instance per test. Also updates the
tests to account for the reality of a dynamic instance number/ID,
rather than the number always being 1.
Points hooks: on update, don't attempt to split hooks multiple times

Also don’t try to split a hook that is just a placeholder.

See #46
Use the correct points type for network post delete points hooks on update

See #46
Split the comment points hook

See #46
Split the comment points hooks on update to 1.4.0

Fixes #46
Split the update unit tests into a separate testcase for each version

Also adds unit tests for comment hook splitting in the 1.4.0 update
test case.

See #46