Add failing test: Tagged caching (driver "redis")

This test passes with cache driver "array" but fails with cache driver "redis". I came across the issue in my own app and double-checked with a fresh vanilla 4.3 install. As per the Redis CLI, the cache values are written but the retrieval seems to fail. I tried but could not find out why. My Redis installation should be alright.

PS: Hope I got the new bug report flow right.