Update open-ended tests to load the module between 'requests'

Under normal operation, XModules are reloaded on each request from a
student. CombinedOpenEnded modules have code that runs at initialization
that validates the students state. These changes makes that code run
during several long-form unit tests (testing CombinedOpenEnded across
multiple 'requests').

These tests are marked as expectedFailure because they now exhibit the
same failures as observed in [LMS-1493] (namely, the students state gets
reset, because CombinedOpenEnded interprets system.ajax_url raising an
error as meaning that the problem definition and the student answers are
in conflict)
Move xmodule_runtime.xmodule_instance registration earlier

This allows XModules (specifically CombinedOpenEnded) to use ajax_url
during their init functions (which would, before, have thrown an
exception).

[LMS-1493]