Create testcases package

[theunrepentantgeek]

What this PR does / why we need it:

To reduce clutter in the astmodel package, creates a new package testcases. Currently we have only a single test case, but that number is going to grow as a part of the implementation for storage variants, so it makes sense to break it out now.

Also fixes a bug where test cases were not being generated for Resources because the interface wasn't being implemented.

Special notes for your reviewer:

How does this PR make you feel:

If applicable:

• this PR contains tests

[codecov-commenter]

Codecov Report

Merging #1557 (d8d56d1) into master (90da966) will decrease coverage by 0.01%.
The diff coverage is 3.07%.

@@            Coverage Diff             @@
##           master    #1557      +/-   ##
==========================================
- Coverage   63.47%   63.46%   -0.02%     
==========================================
  Files         176      176              
  Lines       11694    11697       +3     
==========================================
  Hits         7423     7423              
- Misses       3607     3610       +3     
  Partials      664      664              

Impacted Files	Coverage Δ
hack/generator/pkg/astmodel/object_type.go	73.03% <ø> (ø)
hack/generator/pkg/astmodel/resource_type.go	73.58% <0.00%> (-0.85%)	⬇️
...ack/generator/pkg/astmodel/test_file_definition.go	0.00% <0.00%> (ø)
...g/testcases/object_type_serialization_test_case.go	2.08% <1.69%> (ø)
.../codegen/pipeline_json_serialization_test_cases.go	80.76% <100.00%> (ø)

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