Fix intermittent self-test failures caused by the stats_ext test.

Commit d7f8d26d added new tests to the stats_ext regression test that
included creating a view in the public schema, without realising that
the stats_ext test runs in the same parallel group as the rules test,
which makes doing that unsafe.

This led to intermittent failures of the rules test on the buildfarm,
although I wasn't able to reproduce that locally. Fix by creating the
view in a different schema.

Tomas Vondra and Dean Rasheed, report and diagnosis by Thomas Munro.

Discussion: https://postgr.es/m/CA+hUKGKX9hFZrYA7rQzAMRE07L4hziCc-nO_b3taJpiuKyLLxg@mail.gmail.com