Updating and adding tests for better test coverage

[fstroth]

This PR is still work in progress! Pls don't merge yet!

The goal is to add tests for parts that are not covered atm to prepare for the big changes that will come with the lib restructure.

[codecov]

Codecov Report

Merging #1122 (8cedaea) into master (ba6a714) will increase coverage by 2.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1122      +/-   ##
==========================================
+ Coverage   85.48%   87.64%   +2.15%     
==========================================
  Files         305      305              
  Lines        6675     6675              
==========================================
+ Hits         5706     5850     +144     
+ Misses        969      825     -144     

Flag	Coverage Δ
unittests	87.64% <ø> (+2.15%)	⬆️

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Impacted Files	Coverage Δ
icevision/models/checkpoint.py	83.56% <ø> (+72.60%)	⬆️
icevision/core/class_map.py	76.00% <0.00%> (+2.00%)	⬆️
icevision/core/record_components.py	84.79% <0.00%> (+3.21%)	⬆️
icevision/widgets/__init__.py	100.00% <0.00%> (+100.00%)	⬆️
icevision/widgets/model_choice_ui.py	100.00% <0.00%> (+100.00%)	⬆️

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