chain-test: cover CSV by time

[pinheadmz]

This PR adds tests to check CHECKSEQUENCEVERIFY functionality using the time parameter as well as the height parameter. I'm also not entirely sure the existing tests were totally right. They had CSV OP codes in the outputs of the spending transaction, which is unnecessary, and the fail csv with bad sequence test wasn't actually spending from a CSV output.

I wrote this PR mainly as an exploration of CSV for a cross-chain atomic swap module. The utility function CSVencode is necessary to check against time instead of block height (see BIP68) and this is the function I really wanted to test -- if you want to use time-based CSV in a transaction you'll need to encode the time this way and I don't think there's currently a method anywhere in the library that does this? So actually it might make more sense to put that method somewhere else, like in chain, mtx, or script?

[codecov-io]

Codecov Report

Merging #613 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #613      +/-   ##
==========================================
+ Coverage   53.32%   53.37%   +0.05%     
==========================================
  Files         104      104              
  Lines       27666    27666              
  Branches     4737     4737              
==========================================
+ Hits        14752    14766      +14     
+ Misses      12914    12900      -14

Impacted Files	Coverage Δ
lib/primitives/tx.js	81.51% <0%> (-0.1%)	⬇️
lib/primitives/mtx.js	60.88% <0%> (+0.34%)	⬆️
lib/blockchain/chain.js	61.38% <0%> (+0.83%)	⬆️
lib/script/opcode.js	78.92% <0%> (+1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 125e818...865864a. Read the comment docs.