The test was failing in Ubuntu CI with a 30s timeout. It makes
sense to keep the file so exercise the set allocation logic, but
we can make it shorter.
We were already using OrderedSets in the manager object, but strvs in the
configuration parsing code. Using sets gives us better scaling when many
domains are used.
In oss-fuzz #13059 the attached reproducer takes approximately 30.5 s to be
parsed. Converting to sets makes this go down to 10s. This is not _vastly_
faster, but using sets seems like a nicer approach anyway. In particular, we
avoid the quadratic de-unification operation after each addition.
