In the same spirit as ee7eb0e8, this removes the extra
definition in postgres_test.c.

This is uncovered by building with GCC 10, where -fno-common is the new
default [1][2] (vis a vis -fcommon). I could also reproduce this by
turning on "-fno-common" in older releases of GCC and Clang.

Backpatch to 6X_STABLE.

(cherry picked from commit 108504ff)

Co-authored-by: NMelanie Plageman <mplageman@pivotal.io>

This borrows a page from the playbook of other unittests in order to
regain control after an ereport({ERROR|FATAL} ..) call in order to be
able to perform assertions. By trapping and siglongjmping we can gain
control even after an ERROR/FATAL.

Some tests were already doing this, but it seems nicer to default to using
the real memory manager for all tests. An individual test can still opt-out
by listing the relevant files in <testname>_MOCK_OBJS. Some tests had had
to jump through hoops to mock the memory manager, which is no longer
necessary.