add testing of re-flagging case to FlagHookInvocationsTestCase

FlagHookInvocationsTestCase tests the order in which flag and entity hooks and rules are invoked, and the parameters passed to them.

It tests flagging and unflagging.

We should also test the case where we re-flag an item, by doing $flag->flag('flag', ...) and passing in the $flagging entity.

This should test:

- hook_entity_update() is invoked
- the parameters are correct