Use `ByteToMessageDecoderVerifier` in unit tests

From the review thread:

RedisByteDecoder looks like a fairly complex ByteToMessageDecoder which is okay but please make use of ByteToMessageVerifier to make sure it passes some basic tests. Example use here but there are more in swift-nio and swift-nio-extras. I'm pretty sure I already saw a bug related to partial incoming responses from Redis but ByteToMessageVerifier will test that automatically. Any questions regarding its use, please reach out.

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