Exclude hidden files from example folder during test file collection

[rakshil14-2]

Description of changes:
Tests cases were failing for the users who use Mac Finder that creates hidden system files such as .DS_Store or ._command-name.rst in the directory. Thus, when the test suite is running, it fails because it encounters a unwanted files in the examples folder that has not a .rst or .txt extension.

Resolution
System-generated files like .DS_Store and other hidden files like .command-name.rst are starting with . followed by the file name, hence while gathering all the files having extension other than .rst file, the files starting with . were ignored, making the test suite free from hidden files.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.41%. Comparing base (5f07db1) to head (877c890).
Report is 203 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9530      +/-   ##
===========================================
+ Coverage    93.39%   93.41%   +0.02%     
===========================================
  Files          210      211       +1     
  Lines        16983    17020      +37     
===========================================
+ Hits         15861    15899      +38     
+ Misses        1122     1121       -1     

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[ashovlin]

Change looks good. Please squash before merging, see https://aws.github.io/aws-cli/git_workflows.html#commit-message-formatting for conventions, @kdaily can probably help if you have any questions.