added missing API tests for snapshot endpoints

[mattjackson220]

What does this PR (Pull Request) do?

• This PR fixes No mention of monitoring.json in API documentation for generating a snapshot #5007

This PR adds in API tests that were missing for the snapshot endpoints for all API versions

Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Run the API tests for v1, v2 and v3 and verify that all are successful. Verify that all snapshot endpoints now have API test coverage

If this is a bug fix, what versions of Traffic Control are affected?

The following criteria are ALL met by this PR

No docs, CHANGELOG, or data migrations included since this is just adding missing API tests

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[mitchell852]

@mattjackson220 - not sure if you want to kill 2 birds w/ one stone or a separate PR but this seems kinda related: #5007

[mitchell852]

LGTM