Tests for conditional content (split_test module).

TNL-39
Fix dump course structure when element metadata uses opaque keys.

Make sure that during the serialization, all opaque keys and their
references and serialized using the `unicode` function.

AN-3821
Merge pull request #5654 from edx/christina/visible_graded_content

Tests for conditional content (split_test module).
Merge pull request #5673 from rocha/fix-course-structure-command

Fix dump course structure when element metadata uses opaque keys.