For consistency with the Test.*Context classes for other backends, which live in their own implementation file with their own header, extract TestEGLContext to its own header and TU so that in cases where only a TestEGLContext is required (e.g. EmbedderTestBackingStoreProducerGL), we don't need to include all the various test GL surface classes as well.
GetEGLError is used by both TestEGLContext and the TestGLSurface classes, so moves to its own utils file.
No tests because this is a refactoring with no semantic changes, and the code itself is test code.
Issue: https://github.com/flutter/flutter/issues/158998
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
