Add some tests and improve others to raise coverage percentage. This
should lead to almost 100% for the AbuseFilterParser class. Aside from
this, a couple of changes:
* Remove an unused function
* Let equals_to_any return a genuine result with empty strings
* Remove an if which will never be true in skipOverBraces, since the
function is called after checking the same conditions.
Bug: T201193
Change-Id: I7020b2ed996236c38c5784d161ad98ec44163406
