sigencoding_tests : use pseudorandom generator for flag patterns

Summary:
4096 iterations tests a big mix of flag combination mixes,
and runs far faster than the previous 131072 nonrandom iterations.
The nonrandom approach was getting unsustainable as we are getting up
to 1<<20 in the flag bits (would be 1 million iters).

Test Plan:
I've run this LCG in python to do histogramming & verify the
correct order. Also the iterations include a testpoint to make sure the LCG
is producing the expected sequence.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Maniphest Tasks: T527

Differential Revision: https://reviews.bitcoinabc.org/D2475