no functional tests covering non-OP_TRUE block creation

[instagibbs]

We could use some basic functional tests for block creation with signblockscript set.

1. set signblockscript with a 2-of 3 multisig
2. make block proposals, sign them, combine them, broadcast at various level of signing
3. set signblockscript to a 15-of-15
4. restrict total blocksize, spam blocks full, make sure we only make ones within consensus limits

[instagibbs]

#346 fixes this partially and goes beyond.

we may want to split out the core testing part

[instagibbs]

We have https://github.com/ElementsProject/elements/blob/elements-0.14.1/qa/rpc-tests/feature_blocksign.py now