Improve test coverage for the keep-alive scenario based on HTTP versions within curl transport

[ahsonkhan]

These recently added code-paths here are currently not covered by tests:
https://dev.azure.com/azure-sdk/internal/_build/results?buildId=3775597&view=codecoverage-tab

[ahsonkhan]

@antkmsft, I am assigning this issue to you since you have the most expertise in this space, and introduced the change.

Feel free to unassign if you don't plan to work on it.