`propWithModel` & co do not check if Fwd is kept constant until acked

[rowanG077]

Relevant:

clash-protocols/README.md

Line 84 in febb041

* A circuit driving `Data x` must keep driving the same value until it receives an acknowledgment. This is not yet checked by test harnesses.

Ideally we would check this invariant automatically using the testing framework.

#137 shows that in practice there is/was at least one component that violated this invariant.