Create and reuse a single instance of CosmosClient in Cosmos integration tests

[severussundar]

At the moment, the Cosmos integration tests create a new instance of the CosmosClient for performing operations (such as creating collection, inserting items, etc.) on the cosmos DB. The documentation recommends creating a single instance for the lifetime of the application. Refactoring the tests to create a single instance and re-use that could be an enhancement to the existing cosmos integration tests.

Documentation Link: Azure Cosmos DB Performance Docs