Added tests for validate_sudoku_board.py

[AksharGoyal]

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• 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:

[cclauss]

Closing. This pull request modifies eight different files.

[AksharGoyal]

Hi @cclauss , I am not sure how writing tests for Sudoku affected other files. Can I get some more information on how it happened so that if I open another PR it doesn't affect them?

[cclauss]

The trick is to always create pull requests on a branch other than master.

1. Go to https://github.com/AksharGoyal/Python and click the Sync fork button so your master is up-to-date
2. Go to https://github.com/AksharGoyal/Python/branches and click the New branch button at the upper-right and give the branch a name that makes it clear what the intended change is.

If 1. is not done before 2. the previous edits can creep in.

[AksharGoyal]

I did create a branch this time for adding tests for sudoku but looks like the PR from last time affected this one. I will re-fork and create a branch so this time nothing is affected. Thank you.