Add tests that ensures additional credentialStatus fields can be set in credential passed to /issue

[dlongley]

Additional fields such as refreshService should be able to be set in a credentialStatus field by the caller of /issue. These fields should be preserved and any status index fields should be overwritten.

So, tests should confirm that the status index fields in the newly issued VC are always new and that any other context-defined fields, e.g. refreshService, are preserved.