Update the documentation on how to run tests

[palisadoes]

Is your feature request related to a problem? Please describe.

Our testing documentation needs to be updated to reflect the use of pytest and CodeCov

Describe the solution you'd like

1. Based on this PR
   1. https://github.com/PalisadoesFoundation/switchmap-ng/pull/249/files
2. Update this file
   1. https://github.com/PalisadoesFoundation/switchmap-ng/blob/develop/docs/docs/testing.md
3. It must be updated to show how to:
   1. Run a single test
   2. Run all tests
   3. View the code coverage level of:
      1. All code files
      2. A single file
4. Validate your work is visible in the docusaurus site. The files are in the docs/docs directory.

Describe alternatives you've considered

• N/A

Approach to be followed (optional)

• See above

Additional context

-N/A

Potential internship candidates

Please read this if you are planning to apply for a Palisadoes Foundation internship

• Student Internship Programs talawa#359

[im-vedant]

@palisadoes assign