TestMathlib.sol: Test collateral for a short position and price greater than priceCap.
TestMathlib.sol: Test collateral for a short position and price greater than priceCap.

Verify in testCalculateNeededCollateral that needed collateral for a short position
and price greater than priceCap is 0.

Issue: #27

MarketProject/MarketProtocol,MarketProject/MarketProtocol