Improve mkt.search.tests:TestApi.test_sort (bug 913019)

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, since the real calls are never made, we don't know if ES accepts those sorts params.~~

~~@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 :/~~

Somehow the conversion from tastypie to DRF of the search API seems to have fixed the slowness. I changed my code to do both mocked and unmocked queries, both are useful IMHO.