pgsql: Add test case exercising formerly-unreached code ininheritance_

Add test case exercising formerly-unreached code in inheritance_planner.

There was some debate about whether the code I'd added to remap
AppendRelInfos obtained from the initial SELECT planning run is
actually necessary.  Add a test case demonstrating that it is.

Discussion: https://postgr.es/m/23831.1553873385@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8fba397f0ca7b9a1fd59ab2b676c057dde4f8219

Modified Files
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src/test/regress/expected/updatable_views.out | 57 +++++++++++++++++++++++++++
src/test/regress/sql/updatable_views.sql      | 13 ++++++
2 files changed, 70 insertions(+)