Enable integration testing as if cfnlint is a module

[kddejong]

Issue #, if available:

Description of changes:

• Add testing for cfn-lint used inside other packages

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[kddejong]

Pre-work as I work to rewrite some of the core functionality. This should help with testing and validating that I'm maintaining backwards compatibility.

[codecov]

Codecov Report

Merging #1234 into master will increase coverage by 0.38%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1234      +/-   ##
==========================================
+ Coverage   87.33%   87.72%   +0.38%     
==========================================
  Files         136      136              
  Lines        8062     8062              
  Branches     1984     1984              
==========================================
+ Hits         7041     7072      +31     
+ Misses        598      585      -13     
+ Partials      423      405      -18

Impacted Files	Coverage Δ
src/cfnlint/rules/functions/Sub.py	76.99% <0%> (+0.88%)	⬆️
src/cfnlint/rules/resources/CircularDependency.py	86.17% <0%> (+1.06%)	⬆️
src/cfnlint/rules/resources/properties/JsonSize.py	91.25% <0%> (+1.25%)	⬆️
src/cfnlint/rules/parameters/AllowedValue.py	97.05% <0%> (+1.47%)	⬆️
src/cfnlint/rules/functions/SubNeeded.py	98.43% <0%> (+1.56%)	⬆️
src/cfnlint/rules/__init__.py	88.44% <0%> (+1.59%)	⬆️
...ules/resources/properties/ListDuplicatesAllowed.py	96.72% <0%> (+1.63%)	⬆️
src/cfnlint/rules/resources/ectwo/Ebs.py	85.41% <0%> (+2.08%)	⬆️
...resources/codepipeline/CodepipelineStageActions.py	86.36% <0%> (+2.27%)	⬆️
...rules/resources/codepipeline/CodepipelineStages.py	90.36% <0%> (+2.4%)	⬆️
... and 7 more

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