Directly test combination of contributing 'windows solar' and 'windows conduction' markets

[jtlangevin]

Windows solar and windows conduction markets are combined when preparing ECM markets in 'ecm_prep.py', such that they be considered part of a single window unit for the purposes of ECM competition. It was recently discovered that this combination was not working as intended. The problem was fixed, but the sub-routine for combining these markets in the 'fill_mkts' function should be directly tested.