tox: Stop trying to combine results
tox: Stop trying to combine results

There's nothing to combine, and coverage now fails properly when this
happens. pytest already shows coverage, so there's no need to report
either.

GregBowyer/cld2-cffi,GregBowyer/cld2-cffi,GregBowyer/cld2-cffi,GregBowyer/cld2-cffi,GregBowyer/cld2-cffi