Fix coverage

[teowa]

• prevent the program from crashing if there is an error generating the coverage report.
• add a test that uses local swagger test data to verify that the code is working correctly.
• add a test that expands all PUT and POST models twice and compares the results to each other. This will help to ensure that the expand function is stable and that it is producing consistent results
• fix the discriminator resolve function to support cases where only the child variant is used, and cases where the x-ms-discriminator-value extension is not used
• This task will download the index.zip instead of index.json file and reduce its file size from 50MB to 2MB.

[ms-henglu]

Is it possible to fix the panic instead of calling the cover()?

[teowa]

This is to prevent the main process (which validates and generates the pass report) from crashing if the coverage report fails to generate. The coverage report might fail to generate due to the following reasons:

• The index file cannot be downloaded.
• The online swagger file cannot be downloaded.
• The swagger ref or variant cannot be correctly processed.