Avoid unportable shell syntax in pg_upgrade's test script.

Most of test.sh uses traditional backtick syntax for command substitution,
but commit da9b580d introduced two uses of $(...) syntax, which is not
recognized by very old shells.  Bring those into line with the rest.

Victor Wagner

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

src/bin/pg_upgrade/test.sh

@@ -234,7 +234,7 @@ pg_upgrade $PG_UPGRADE_OPTS -d "${PGDATA}.old" -D "${PGDATA}" -b "$oldbindir" -B
 # Windows hosts don't support Unix-y permissions.
 case $testhost in
 	MINGW*) ;;
-	*)	if [ $(find ${PGDATA} -type f ! -perm 640 | wc -l) -ne 0 ]; then
+	*)	if [ `find ${PGDATA} -type f ! -perm 640 | wc -l` -ne 0 ]; then
 			echo "files in PGDATA with permission != 640";
 			exit 1;
 		fi ;;
@@ -242,7 +242,7 @@ esac
 
 case $testhost in
 	MINGW*) ;;
-	*)	if [ $(find ${PGDATA} -type d ! -perm 750 | wc -l) -ne 0 ]; then
+	*)	if [ `find ${PGDATA} -type d ! -perm 750 | wc -l` -ne 0 ]; then
 			echo "directories in PGDATA with permission != 750";
 			exit 1;
 		fi ;;