Add PostgreSQL promotion database gate

tests/test-promote-database-shadow.sh

@@ -12,8 +12,8 @@ grep -q 'comparison=MATCH' "$FILE"
 grep -q 'promote.database shadow MISMATCH' "$FILE"
 
 SHADOW_LINE="$(
-    grep -n '^[[:space:]]*mc_pg_promote_database_shadow$' "$FILE" |
-    tail -1 |
+    grep -n 'if ! mc_pg_promote_database_shadow; then' "$FILE" |
+    head -1 |
     cut -d: -f1
 )"