add redis cluster client tests

[leeym]

I should have done this in #48, but I think it is not too late to do it now.

The existing test environment only has single node Redis and there is no Redis cluster.
Given that the Redis and Redis cluster may behave differently, I think we should test them separately.

• keep Redis at port 6379 while adding Redis cluster at port 7000-7005
• whenever we test Redis, we test Redis cluster as well

Also run it in two builds:

• remove hash tags in redisKey to reproduce the error CROSSSLOT Keys in request don't hash to the same slot
• add hash tags back in redisKey to show that it works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.05%. Comparing base (9d9ccea) to head (00de771).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #50   +/-   ##
=======================================
  Coverage   80.05%   80.05%           
=======================================
  Files          12       12           
  Lines        1449     1449           
=======================================
  Hits         1160     1160           
  Misses        212      212           
  Partials       77       77           

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[mennanov]

Awesome! Thanks!