Test for allmydata.immutable.layout.WriteBucketProxy invariant

[itamarst]

#3915 adds an invariant that calculated length must match the written data; this is sensible (and required by HTTP!) but wasn't previously enforced, relying on Foolscap write's semantics that allowed creating holes with nulls.

It would be nice to be able to test this (and other invariants) but as is the code is factored in ways that make it very difficult to test without creating a whole pile of different objects. This may be the influence of Foolscap pushing for more RPC-ish structure?

Anyway writing this test is going to require some Refactoring... For now this is just enforced at runtime.