Additional API endpoints: /test endpoints

To achieve parity with the API endpoints that Maven offers, there are additional endpoints that we need to add.This issue focuses on the testing endpoints and aims to add the following endpoints:

Route	Notes
GET /virtual_registries/container/upstreams/:id/test	Test an existing upstream connection
POST /virtual_registries/container/upstreams/test	Test the connection before an upstream is created

References:

• https://docs.gitlab.com/api/maven_virtual_registries/#test-connection-to-an-upstream-registry
• https://docs.gitlab.com/api/maven_virtual_registries/#test-connection-before-creating-an-upstream-registry

After the endpoints are added, update the API documentation.