Remove mocks in mkt.search.tests.test_api:TestApi.test_sort

This test has been slow for ages, and now it's unusable on my machine, and results in "MySQL server has gone away" in jenkins.

From my testing, those mocks actually slow us down more than just calling ES, somehow it ends up eating a lot of memory too. Also, this hides problems with DEFAULT_SORTING.

@robhudson what do you think? Not 100% happy with it because we don't know for sure that the sort parameter gets passed down to ES but I couldn't pinpoint the exact source of the issues with the mocks :/