Support testing of cases where table schemas change after planning.

We have various cases where we allow DDL on tables to be performed with
less than full AccessExclusiveLock.  This requires concurrent queries
to be able to cope with the DDL change mid-flight, but up to now we had
no repeatable way to test such cases.  To improve that, invent a test
module that allows halting a backend after planning and then resuming
execution once we've done desired actions in another session.  (The same
approach could be used to inject delays in other places, if there's a
suitable hook available.)

This commit includes a single test case, which is meant to exercise the
previously-untestable ExecCreatePartitionPruneState code repaired by
commit 7a980dfc6.  We'd probably not bother with this if that were the
only foreseen benefit, but I expect additional test cases will use this
infrastructure in the future.

Test module by Andy Fan, partition-addition test case by me.

Discussion: https://postgr.es/m/20200802181131.GA27754@telsasoft.com

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