Use cite-group-delimiter as fallback for year-suffix-delimiter

[Drodt]

The CSL spec is a bit unclear about this, but it says about cite-group-delimiter:

Activates cite grouping and specifies the delimiter for cites within a cite group. Defaults to “, “. E.g. with delimiter on cs:layout in cs:citation set to “; “, collapse set to “year”, and cite-group-delimiter set to “,”, citations look like “(Doe 1999,2001; Jones 2000)”.

It is reasonable and expected by the tests that this extends to year suffixes.

Makes 2 tests pass

[PgBiel]

Thanks!