Move the sanity check in udata SerializeSize() to a test

[kcalvinalvin]

The code snippet here is responsible for a lot of memory allocations.

https://github.com/mit-dci/utreexo/blob/259fc1a7bf87a6931de63dde1ab4b66fbd1ea6b4/btcacc/udata.go#L124-L129

We can instead move this to a test so that we don't have this overhead for the binary