UnitTest for blocking PubSub auto-reconnect

[kristjanvalur]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

_NOTE: Maybe this requires us to clarify the API, for it appears to be the intention that connections are automatically re-established.

Description of change

Issue #2089 is about automatic reconnection in async PubSub.
This PR adds unittests verifying how the connection in the synchronous PubSub will automatically reconnect.
The caller needs merely catch redis.ConnectionError and then retry the read operations and a new connection
will be established.

Two tests are provided, one which calls disconnect() on the underlying connection (and does not raise an error in the
reader) and one which mocks a socket.error coming out of the socket.recv() call.

[codecov-commenter]

Codecov Report

Merging #2256 (a33a3a6) into master (e6cd4fd) will decrease coverage by 0.04%.
The diff coverage is 98.55%.

@@            Coverage Diff             @@
##           master    #2256      +/-   ##
==========================================
- Coverage   91.85%   91.81%   -0.05%     
==========================================
  Files         108      108              
  Lines       27783    27852      +69     
==========================================
+ Hits        25521    25572      +51     
- Misses       2262     2280      +18     

Impacted Files	Coverage Δ
tests/test_pubsub.py	99.58% <98.55%> (-0.18%)	⬇️
tests/test_asyncio/test_bloom.py	94.57% <0.00%> (-1.36%)	⬇️
tests/test_bloom.py	98.24% <0.00%> (-1.32%)	⬇️
redis/asyncio/connection.py	83.18% <0.00%> (-0.89%)	⬇️
redis/commands/bf/commands.py	98.29% <0.00%> (-0.57%)	⬇️
tests/test_asyncio/test_search.py	98.35% <0.00%> (-0.33%)	⬇️
redis/connection.py	86.42% <0.00%> (ø)
tests/test_cluster.py	96.96% <0.00%> (ø)

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[dvora-h]

@kristjanvalur See test failure

[kristjanvalur]

Closing this PR in favour of another, (#2281) with better unit tests and suggested fixes.