Add tests

[nikhil-prabhu]

The crate is currently manually tested for correctness. While manual testing is required in our case to ensure that the providers are correctly detected in real-world scenarios, unit tests should also be added for testing functional and logical correctness.

Current blockers

• The methods that check the provider metadata servers make real HTTP requests to third party URLs. We need a way to mock/patch this as these requests won't work in unit tests.
• The methods that check the provider vendor files must be mocked/patched as the vendor files won't actually exist in our test environment.
• mockall seems like a good option for now for mocking. However, writing unit tests using mocks doesn't provide code coverage.
• guerrilla seems promising for monkey patching our methods (and also provides code coverage), but doesn't seem to be working fully as expected in my limited testing.