In many embedder tests, we want to get at the appropriate backend-specific `EmbedderTestContext` subclass (`EmbedderTestContextGL`, etc.) in order to make backend-specific setup calls such as `SetGLFBOCallback()` or others. Formerly, this required casting the returned `EmbedderTestContext&` to the appropriate subclass in each test.
This templatises the `GetEmbedderContext()` method to return the appropriate backend-specific subclass directly.
Issue: https://github.com/flutter/flutter/issues/158998
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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