Test verifying that ChunkedEncodingError is raised on an invalid Transfer-Encoding: chunked response.

[theGOTOguy]

This logic is not currently exercised anywhere in requests' tests.

For context, most errors that occur during the process of handling a request can be retried through urllib3's Retry. Currently, chunked encoding bypasses this, leading to a situation where users of requests are left to roll their own custom error handling for chunked encoding. #5664 seeks to remedy that, but there are two issues:

First, the current logic around chunked encoding has limited tests, and there is concern that accepting #5664 would introduce a regression.
Second, switching from requests' internal exceptions to urllib3's exceptions is a breaking change that will require appropriate notification to existing users.

This is a first step towards addressing both issues by exercising the chunked encoding logic internally, introducing a test that will break when requests switches to using urllib3's exceptions for chunked encoding, and providing a simple example to start with for any who seek to write further tests for the chunked encoding logic.

[nateprewitt]

Hey @theGOTOguy, thanks for the contribution. I think the concern of changing from our current chunked implementation to urllib3 is more with request serialization rather than response deserialization.

That said, you're right we're not testing this behavior anywhere when we probably should. We can merge this as a general testing improvement.

[theGOTOguy]

I appreciate the feedback on what tests are needed, @nateprewitt. I will look into request serialization tests next.