pgsql: postgres_fdw: Fix test that didn't test what it claimed.

postgres_fdw: Fix test that didn't test what it claimed.

Antonin Houska reported that the planner does consider pushing
postgres_fdw_abs() to the remote side, which happens because we make
it shippable earlier in the test case file.

Jeevan Chalke provided this patch, which changes the join
condition to use random(), which is not shippable, instead.
Antonin reviewed the patch.

Discussion: http://postgr.es/m/15265.1511985971@localhost

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/950222780535e6d2ea560cd8b3db5308652ddd42

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 24 +++++++++++++-----------
contrib/postgres_fdw/sql/postgres_fdw.sql | 2 +-
2 files changed, 14 insertions(+), 12 deletions(-)