Backend: Add isAvailable test
Enable: Add more tests for full code coverage
KeyShortenTest: Use random long key

Otherwise it could hide errors with trimming the key if it's just an 'a'
repeated many times.
KeyShorten: Add test for changeKeys
Local: Call through to setMultiple

This lost the single setMultiple call by calling set once for each key.
Local: Add getMultiple test

To make sure that hits are merged into the local cache.
Backend: Add test for negative (in|de)crement
MultiScope: Add test for addScope