Points component: clean the caps of all blogs on activation

Formerly we only cleaned the caps of the current blog if the plugin
wasn’t network active. Now we remove them either way.

See #28
AJAX unit tests: improve network activation support

See #33
Uninstall tests: clear the roles cache

The fact that the roles “cache” wasn’t being cleared properly was
causing our build to fail.

Also makes the related cap assertion more semantic.

See #28