Further portability hacking in pg_upgrade's test script.

I blew the dust off a Bourne shell (file date 1996, yea verily) and
tried to run test.sh with it.  It mostly worked, but I found that the
temp-directory creation code introduced by commit be76a6d39 was not
compatible, for a couple of reasons: this shell thinks "set -e" should
force an exit if a command within backticks fails, and it also thinks code
within braces should be executed by a sub-shell, meaning that variable
settings don't propagate back up to the parent shell.  In view of Victor
Wagner's report that Solaris is still using pre-POSIX shells, seems like
we oughta make this case work.  It's not like the code is any less
idiomatic this way; the prior coding technique appeared nowhere else.

(There is a remaining bash-ism here, which is that $RANDOM doesn't do
what the code hopes in non-bash shells.  But the use of $$ elsewhere in
that path should be enough to ensure uniqueness and some amount of
randomness, so I think it's okay as-is.)

Back-patch to all supported branches, as the previous commit was.

Discussion: https://postgr.es/m/20180720153820.69e9ae6c@fafnir.local.vm

contrib/pg_upgrade/test.sh

@@ -42,20 +42,18 @@ case $testhost in
 		# script; the outcome mimics pg_regress.c:make_temp_sockdir().
 		PGHOST=$PG_REGRESS_SOCK_DIR
 		if [ "x$PGHOST" = x ]; then
-			{
-				dir=`(umask 077 &&
-					  mktemp -d /tmp/pg_upgrade_check-XXXXXX) 2>/dev/null` &&
-				[ -d "$dir" ]
-			} ||
-			{
+			set +e
+			dir=`(umask 077 &&
+				  mktemp -d /tmp/pg_upgrade_check-XXXXXX) 2>/dev/null`
+			if [ ! -d "$dir" ]; then
 				dir=/tmp/pg_upgrade_check-$$-$RANDOM
 				(umask 077 && mkdir "$dir")
-			} ||
-			{
-				echo "could not create socket temporary directory in \"/tmp\""
-				exit 1
-			}
-
+				if [ ! -d "$dir" ]; then
+					echo "could not create socket temporary directory in \"/tmp\""
+					exit 1
+				fi
+			fi
+			set -e
 			PGHOST=$dir
 			trap 'rm -rf "$PGHOST"' 0
 			trap 'exit 3' 1 2 13 15