tests: add test for DefaultAuthorizers not a string in API

[jfuss]

In issue #1245, we had a cases were SAM would fail due to improper validation.
In updating #1286, I noticed this was patched in #1774 but we only added tests
for AWS::Serverless::StateMachine. This commit adds an additional test to cover
AWS::Serverless::Api.

Issue #, if available:
n/a but is a test to cover #1245

Description of changes:
Adds additional test

Description of how you validated changes:
make pr ran successfully

Checklist:

• Add/update unit tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• Add/update integration tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #2313 (bdf5286) into develop (e7a1496) will increase coverage by 0.89%.
The diff coverage is 98.20%.

@@             Coverage Diff             @@
##           develop    #2313      +/-   ##
===========================================
+ Coverage    93.58%   94.47%   +0.89%     
===========================================
  Files           90       97       +7     
  Lines         6124     7061     +937     
  Branches      1260     1435     +175     
===========================================
+ Hits          5731     6671     +940     
+ Misses         183      177       -6     
- Partials       210      213       +3     

Impacted Files	Coverage Δ
samtranslator/model/api/api_generator.py	94.38% <92.95%> (+0.02%)	⬆️
samtranslator/__init__.py	100.00% <100.00%> (ø)
samtranslator/feature_toggle/dialup.py	100.00% <100.00%> (ø)
samtranslator/feature_toggle/feature_toggle.py	100.00% <100.00%> (+12.16%)	⬆️
samtranslator/intrinsics/actions.py	98.78% <100.00%> (+0.06%)	⬆️
samtranslator/intrinsics/resource_refs.py	95.83% <100.00%> (-0.17%)	⬇️
samtranslator/metrics/method_decorator.py	100.00% <100.00%> (ø)
samtranslator/metrics/metrics.py	100.00% <100.00%> (ø)
samtranslator/model/__init__.py	97.63% <100.00%> (-0.02%)	⬇️
samtranslator/region_configuration.py	77.77% <0.00%> (-22.23%)	⬇️
... and 40 more

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