test: refactor instrumentation/redis tests

[zvkemp]

As discovered in #1370, the arrangement of redis tests is prone to mistakes — they are grouped perhaps correctly as unit tests, however, the implementation of the target libraries makes this complicated:

• redis (~> 4): uses the RedisV4Client patches
• redis-client: uses the RedisClient middleware instrumentation
• redis (~> 5): a wrapper around RedisClient; uses the RedisClient middleware(s).

In most cases, the actions and expectations should be the same (but there are some edge cases). It may be worthwhile to group the tests based on their target gem rather than the instrumentation used.