Add test for split varint decoding

[jaskij]

An additional test to force testing non-contguous chunks in Buf. This is necessary, since this edge case is, theoretically, supported by the Buf trait.

Additionally, this also serves to test the slow path decoding.

Although, as https://github.com/tokio-rs/bytes/issues/474 shows, this is probably a low-priority case.

Fixes #559