Update mapping tests for misc. extensions

[mbabker]

• Refactors the reference integrity mapping test to use an annotated document instead of YAML mapping (which is long unsupported in the MongoDB ODM now)
• Adds references mapping tests (only covering annotation/attribute) with both an ORM mapped entity and an ODM mapped document to try and cover both sides of those references
• Refactors the tree mapping test to use XML fixtures instead of YAML
• Removes some duplicate XML specific test cases since this should be covered by the general mapping tests (or cases added for both annotation/attribute and XML)