Add tests for limit$, skip$ and sort$

This code comes from the implementation of my seneca-oracle-store and it's aimed to assert that limit$, skip$ and sort$ are implemented and work correctly.

I grouped the tests into two different blocks, thay need to be manually added to the store tests (see https://github.com/paolochiodi/seneca-store-test/compare/rjrodger:master...master#diff-6a501d027bacc79774112b644de665dbR22 )

I tested the following stores against these new tests:
- mem-store: all green
- mongo-store: `listlimit5skip2` is failing
- oracle-store: all green
- mysql-store: fail badly (looks like metaqueries aren't supported: I plan to implement them today)

ps:  I've another set of tests I'd like to share (includes removing with metaqueries), I'll open another pull request later today or in the next days