pg_upgrade: Make test.sh's installcheck use to-be-upgraded version's bindir.

On master (after 700538) the old version's installed psql was used -
even when the old version might not actually be installed / might be
installed into a temporary directory. As commonly the case when just
executing make check for pg_upgrade, as $oldbindir is just the current
version's $bindir.

In the back branches, with --install specified, psql from the new
version's temporary installation was used, without --install (e.g for
NO_TEMP_INSTALL, cf 47b3c266), the new version's installed psql was
used (which might or might not exist).

Author: Andres Freund
Discussion: https://postgr.es/m/20190522175150.c26f4jkqytahajdg@alap3.anarazel.de

src/bin/pg_upgrade/test.sh

@@ -76,6 +76,14 @@ temp_root=`pwd`/tmp_check
 oldsrc=`cd "$oldsrc" && pwd`
 newsrc=`cd ../../.. && pwd`
 
+# We need to make pg_regress use psql from the desired installation
+# (likely a temporary one), because otherwise the installcheck run
+# below would try to use psql from the proper installation directory
+# of the target version, which might be outdated or not exist. But
+# don't override anything else that's already in EXTRA_REGRESS_OPTS.
+EXTRA_REGRESS_OPTS="$EXTRA_REGRESS_OPTS --bindir='$oldbindir'"
+export EXTRA_REGRESS_OPTS
+
 # While in normal cases this will already be set up, adding bindir to
 # path allows test.sh to be invoked with different versions as
 # described in ./TESTING