Improve testing of containers

[rly]

assertContainerEqual does not check that the name, container_source, parent, or children match - just that the other fields (which does not include name!) matches. This PR fixes that.

[codecov]

Codecov Report

Merging #217 into dev will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              dev    #217      +/-   ##
=========================================
+ Coverage   70.38%   70.4%   +0.01%     
=========================================
  Files          31      31              
  Lines        5920    5920              
  Branches     1389    1389              
=========================================
+ Hits         4167    4168       +1     
+ Misses       1322    1321       -1     
  Partials      431     431

Impacted Files	Coverage Δ
src/hdmf/common/sparse.py	71.42% <0%> (+2.85%)	⬆️

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[rly]

A new change:
The unittest framework automatically tries to run abstract TestCase classes as tests. These end up getting skipped. Instead, I converted those classes to abstract mix-in classes. Actual test classes inherit from the mix-in class to use the mix-in method definitions, and they will need to override unimplemented methods. See https://stackoverflow.com/questions/4566910/abstract-test-case-using-python-unittest

This design pattern is now used in several places, most notably the change from TestMapRoundTrip to TestH5RoundTripMixin