pgsql: Force hash joins to be enabled in the hash join regressiontests

Force hash joins to be enabled in the hash join regression tests.

Otherwise the regressplans.sh tests generate extremely slow nested
loop joins.  Back-patch to 11 where the hash join tests came in.

Reported-by: Michael Paquier
Discussion: https://postgr.es/m/20190708055256.GB2709%40paquier.xyz

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3955c50f3762be6f0a63573c5b4efef72b30dc2b

Modified Files
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src/test/regress/expected/join.out | 1 +
src/test/regress/sql/join.sql      | 1 +
2 files changed, 2 insertions(+)