Want to unconditionally create Valid{} packets over given buffer

[FelixMcFelix]

We have a few places in tests where we need to preset values in a buffer in order to successfully create a zero-copy view for a given packet type if the length computation of variable length elements depends on existing state -- for instance, Ipv6 packets. It would be useful in such cases to bypass nested subparsing, or to force a minimum-sized repr in a given buffer.