test: add IntroSort tests

[crhallberg]

• move demo1 and demo2 into test
• cover obvious code branches

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?
• Add a test for existing algorithm

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new JavaScript files are placed inside an existing directory.
• All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames.
  Example:UserProfile.js is allowed but userprofile.js,Userprofile.js,user-Profile.js,userProfile.js are not
• All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.43%. Comparing base (08d8c6b) to head (109f32d).

Additional details and impacted files

@@ Coverage Diff @@ ## master #1862 +/- ## ========================================== + Coverage 85.91% 87.43% +1.52%  ========================================== Files 379 379 Lines 19778 19713 -65 Branches 3015 3062 +47 ========================================== + Hits 16993 17237 +244  + Misses 2785 2476 -309 

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[crhallberg]

Code Coverage for IntroSort.js is now 100%.