Adding the doctests And Fixing the code in longest_sub_array

[AasheeshLikePanner]

Describe your change:

Adding Doctests in longest_sub_array.py. Fixing the code. The code before takes a String as an input and return output. I Change the code to take the output as a list and return an int.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• [] Documentation change?

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 Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• [] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".
  Improve our test coverage #9943