WIP: Add a raw_property_data() method to Service and local.Store

This method takes a commit, an object and a raw property name and
returns the data of the raw property.

TODO: This method should check for whether the property is raw or
not and fail if it isn't.
Make run_python_test able to parse multiple YAML documents

The first YAML document is assigned to the same test variable as
before (output_yaml), the second to output_raw_properties, which
is expected to be a dictionary mapping object UUIDs to dictionaries
mapping raw property names to raw property data:

    <uuid 1>:
      foo: <raw data>
      bar: <raw data>
    <uuid 2>:
      foo: <raw data>

This can then be used to verify that specific raw properties hold
expected raw data.
Print raw props along with objects from the latest commit of a ref

This is required so that scenario tests can verify that
update-raw-property and unset-raw-property actions modify raw
properties as expected.
Add more scenario tests for raw property updates