Add unit tests

[sevein]

Include tests/unit to cover some areas that are not easily tested from tests/integration.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 61.53846% with 5 lines in your changes missing coverage. Please review.

Project coverage is 86.60%. Comparing base (98f52a9) to head (890123b).

Files with missing lines	Patch %	Lines
clamav_client/clamd.py	60.00%	4 Missing ⚠️
clamav_client/scanner.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   84.68%   86.60%   +1.91%     
==========================================
  Files           3        3              
  Lines         320      321       +1     
  Branches       37       37              
==========================================
+ Hits          271      278       +7     
+ Misses         32       29       -3     
+ Partials       17       14       -3     

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