Implement unit test for whitelist_markets and blacklist_markets

[theoreticalbts]

The whitelist_markets and blacklist_markets are properties on an asset. But first we must ask, what are the intended semantics of these fields?

Assigning to @bytemaster to answer this question.

[bytemaster]

These fields limit which markets an asset can be traded against.

Whitelist markets means a set of other assets that a user issued asset may trade against.
Blacklist markets implies the asset can trade against anything but the specified markets.

[vikramrajkumar]

This issue was moved to bitshares/bitshares-core#47