Fix env vars when macro is used without extensionLayerVersion set

[sfirrin]

What does this PR do?

• Fixes a bug where not all env vars would be set if the macro is used without the extension
• Fixes existing tests
• Adds unit tests

Motivation

https://github.com/DataDog/datadog-cloudformation-macro/issues/92 thanks to @cscetbon

Testing Guidelines

• Added unit tests
• Tested with an example stack

Additional Notes

Types of changes

• [x] Bug fix
• [ ] New feature
• [ ] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[codecov-commenter]

Codecov Report

Merging #94 (c42237d) into master (2e87074) will decrease coverage by 0.03%. The diff coverage is 57.14%.

:exclamation: Current head c42237d differs from pull request most recent head e8de585. Consider uploading reports for the commit e8de585 to get more accurate results

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   86.46%   86.43%   -0.03%     
==========================================
  Files           8        8              
  Lines         613      612       -1     
  Branches      154      153       -1     
==========================================
- Hits          530      529       -1     
  Misses         72       72              
  Partials       11       11              

Impacted Files	Coverage Δ
serverless/src/env.ts	75.77% <57.14%> (-0.15%)	:arrow_down:

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