ChainSync client test: decrease number of tests without any sent headers

[amesgen]

Upon inspection, I was surpsised that ~70% of the cases had SomeNotEarly_SomeEarly Zero Zero, which implies the test never involved downloading a header!

My initial guess is that the client and server chain are diverging before they ever reach startTick.

I suspect we want to tune the generator to visit that less often. But that can be a separate Issue.

Originally posted by @nfrisby in #525 (comment)

Yeah, that seems be the case, when always setting startTick to 1, the Zero Zero case shrinks to ~9% of all cases.