pgsql: Fix intermittent self-test failures caused by the stats_exttest

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

Commit d7f8d26d9 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3d9a3ef5cbfc70bd2802c3f0da3fbc4e5abdf377

Modified Files
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src/test/regress/expected/stats_ext.out | 42 ++++++++++++++++++---------------
src/test/regress/sql/stats_ext.sql      | 40 ++++++++++++++++---------------
2 files changed, 44 insertions(+), 38 deletions(-)