TST: Added tests for get_turnover
TST: Added tests for get_turnover

This added a simple test case for get_turnover.
1. No transactions -> no turnover
2. 100% value traded daily -> 50% daily turnover

This was repeated with a monthly freq, I used 20 days
so the monthly result should be the sum of the daily result.

YihaoLu/pyfolio,chayapan/pyfolio,femtotrader/pyfolio,r0k3/pyfolio,femtotrader/pyfolio,quantopian/pyfolio,ChinaQuants/pyfolio,raybuhr/pyfolio,r0k3/pyfolio,sketchytechky/pyfolio,yanheven/pyfolio,dursk/pyfolio,YihaoLu/pyfolio,yanheven/pyfolio,raybuhr/pyfolio,ChinaQuants/pyfolio,sketchytechky/pyfolio,quantopian/pyfolio,dursk/pyfolio