ReferenceTest : add tests that make sure metadata is preserved on promoted plugs
ReferenceTest : add tests that make sure metadata is preserved on promoted plugs

As part of this, we need to test the new edit tracking functionality and whether
metadata gets serialised correctly for plugs on References.

ImageEngine/gaffer,hradec/gaffer,boberfly/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,boberfly/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,boberfly/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,hradec/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,hradec/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,boberfly/gaffer,hradec/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer