Partitioning tuple route code assumes that the partition chosen while
descending the partition hierarchy is always the correct one.  This is
true except when the partition is the default partition and another
partition has been added concurrently: the partition constraint changes
and we don't recheck it.  This can lead to tuples mistakenly being added
to the default partition that should have been rejected.

Fix by rechecking the default partition constraint while descending the
hierarchy.

An isolation test based on the reproduction steps described by Hao Wu
(with tweaks for extra coverage) is included.

Backpatch to 12, where this bug came in with 898e5e32.

Reported by: Hao Wu <hawu@vmware.com>
Author: Amit Langote <amitlangote09@gmail.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/CA+HiwqFqBmcSSap4sFnCBUEL_VfOMmEKaQ3gwUhyfa4c7J_-nA@mail.gmail.com
Discussion: https://postgr.es/m/DM5PR0501MB3910E97A9EDFB4C775CF3D75A42F0@DM5PR0501MB3910.namprd05.prod.outlook.com