[WIP] Test table coverage

I've added a few tests just so you guys can see my progress. I hope to have it fully tested today or tomorrow.
I remember why I had trouble with this now. 

So, TableSymbol automatically declares iscolumn=False, just by virtue of it being a representation of a table. So in order to test this I would need to create a new object that uses the TableExpr iscolumn property, merely for the purposes of testing. 

Does pytest have a particular way of doing this?