Add more tests for v2, v3, and v4 container metadata endpoints

[amogh09]

Summary

• Add more tests for v2, v3, and v4 container metadata endpoints. Currently there are many error cases that are not covered by existing tests. This change fills that gap in coverage.
• General improvements to the test file.

Implementation details

• Expected responses for v4 container metadata in task_server_setup_test.go are currently initialized with v2.ContainerResponse.Network field that must be set to nil before the struct is used for assertions. Fix this by setting the field to nil when the struct is initialized.
• Add a generic testTMDSRequest function that may be used to test happy and error cases of all metadata endpoints. This function removes the need to set up TMDS, setup mocks, perform JSON unmarshalling, and perform assertions in each test function, thus greatly simplifying TMDS testing.
• Add TestV2ContainerMetadata, TestV3ContainerMetadata, and TestV4ContainerMetadata functions for testing v2, v3, and v4 container metadata endpoints, respectively. These test functions include all happy and error cases. Remove existing tests for these endpoints as they are covered in the new test functions.

Testing

New tests cover the changes: yes

Description for the changelog

Add more tests for v2, v3, and v4 container metadata endpoints

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.