Add unittest for PubSub.connect()

[kristjanvalur]

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Description of change

Issue #2089 demonstrates how it was difficult to keep a PubSub loop running in face of ConnectionError since the
message getting methods don't automatically reconnect.

After PR #2148 it is now possible to call PubSub.connect() to refresh connectivity as part of a handling loop.

This PR adds a unittest which tests that such a loop successfully reconnects after being disconnected.

[codecov-commenter]

Codecov Report

Merging #2167 (994da43) into master (5c99e27) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2167      +/-   ##
==========================================
+ Coverage   91.98%   92.01%   +0.02%     
==========================================
  Files         105      105              
  Lines       25287    25334      +47     
==========================================
+ Hits        23260    23310      +50     
+ Misses       2027     2024       -3     

Impacted Files	Coverage Δ
tests/test_asyncio/test_pubsub.py	99.40% <100.00%> (+0.06%)	⬆️
tests/test_asyncio/test_search.py	98.35% <0.00%> (-0.33%)	⬇️
tests/test_cluster.py	97.48% <0.00%> (ø)
redis/asyncio/connection.py	84.83% <0.00%> (+0.11%)	⬆️
redis/asyncio/client.py	92.85% <0.00%> (+0.69%)	⬆️

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[dvora-h]

Thanks @kristjanvalur! Great to see improvements for our tests