chore: actually exclude test files from being installed

[yan12125]

Issue #, if available: N/A

Description of changes:

In Arch Linux, we build python-aws-sam-translator package from sources. Before this change, test files are installed:

$ python setup.py install --root=arch-pkgdir
$ ls arch-pkgdir/usr/lib/python3.7/site-packages/tests
intrinsics  model  plugins  policy_template_processor  sdk  swagger  translator

This is a problem as the directory /usr/lib/python3.7/site-packages/tests conflicts with other packages.

Description of how you validated changes:

After this change, there are no longer test files

$ python setup.py install --root=arch-pkgdir
$ ls arch-pkgdir/usr/lib/python3.7/site-packages
aws_sam_translator-1.15.0-py3.7.egg-info  samtranslator

Checklist:

• Write/update tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected
• Add/update example to examples/2016-10-31

I think these checks are irrelevant are this is a packaging fix.

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

[codecov-io]

Codecov Report

Merging #1132 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop   #1132   +/-   ##
=======================================
  Coverage     94.8%   94.8%           
=======================================
  Files           71      71           
  Lines         3486    3486           
  Branches       680     680           
=======================================
  Hits          3305    3305           
  Misses          93      93           
  Partials        88      88

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[jlhood]

Thanks for fixing this!