Commit c855872074 introduced a new parameter to pg_regress to set
the directory where to look for expected files, but accidentally
only implemented it for when compiling pg_regress for ECPG tests.
Fix by adding support for the parameter to the main regression test
compilation of pg_regress as well.
Backpatch to v16 where --expecteddir was introduced.
Author: Anthonin Bonnefoy <anthonin.bonnefoy@datadoghq.com>
Discussion: https://postgr.es/m/CAO6_Xqq5yKJHcJsq__LPcKwSY0XHRqVERNWGxx5ttNXXj7+W=A@mail.gmail.com
Backpatch-through: 16
5643262999