Use settings.DANDI_DOI_API_PREFIX instead of 10.80507 in tests

Where does that hardcoded DOI prefix come from? Should it be parameterized somehow?

I realize this is a testing module, so maybe it doesn't matter here, but if this value corresponds to a production datacite instance or something like that, I wonder if it should be stored in and retrieved from an environment variable or something like that.

_Originally posted by @waxlamp in https://github.com/dandi/dandi-api/pull/467#discussion_r686293117_

`10.80507` is the test datacite ID for DANDI, which is what we should be using. It should be parameterized though instead of being hardcoded in tests everywhere.

I think we will need to add `DANDI_DOI_API_PREFIX=10.80507` to the test configuration as well.

However, it is very important that we don't specify all of the `DANDI_DOI_*` fields. If they are all present, a new draft DOI will be minted every time tox runs a publish test.