test: Add test fixtures & implement testDatasource for tag

[mure]

Pull Request

🤨 Rationale

Writing unit tests for data sources is a little tricky because requests are made by the BackendSrv instance retrieved from a getBackendSrv() call. This PR adds some fixtures to make test specs cleaner and implements the tag's testDatasource method as an example.

👩‍💻 Implementation

I really wanted to use Mock Service Worker, so that we could mock routes at the network level, but the BackendSrv implementation is apart of Grafana's unpackaged code, so we have to mock it out.

Previously, we were using jest.mock to monkey patch the @grafana/runtime module, but it only applies to the file that called it, so it's difficult to refactor. Instead, we can just inject the BackendSrv in DataSource's constructors. Then jest-mock-extended does a lot of the heavy lifting, letting us create a type-safe mock of the BackendSrv interface. The mock even comes with a calledWith() extension, so we can verify that the correct route is called.

🧪 Testing

This is self-explanatory 😄

✅ Checklist

• This PR has a title that follows the commit message format.

[mure]

🎉 This PR is included in version 1.6.1 🎉

The release is available on:

• v1.6.1
• GitHub release

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