Split/perf test

@nasthagiri @dianakhuang please review (@cpennington feel free to review at your discretion)

The first commit fixes a problem in split mongo where it wasn't properly serializing fields. The test in the last commit tests that code but I should probably add a test to ensure it properly deserializes. (I'll do that now).

The 4th commit (https://github.com/edx/edx-platform/commit/732940494584050e03796ee6381b2588bb90b040) fixes the test contextmanager.

commit (https://github.com/edx/edx-platform/commit/d5da3cf9f8dfd9cda78510654de04e4bd37a23c1) adds a hopefully useful test creation harness w/ a copy of toy created via modulestore commands rather than import and configurations for supporting split as well as mongo.
Yes, the xml course invocations don't add to the call count (give 0 calls); so, I had to split out the calls to not iterate over all stores.