Use pg_is_in_recovery() instead of relying on transaction_read_only GUC variable in promote() func

Author: zilder

testgres/node.py

@@ -713,11 +713,11 @@ def promote(self, dbname=None, username=None):
         # for versions below 10 `promote` is asynchronous so we need to wait
         # until it actually becomes writable
         if self._pg_version < '10':
-            check_query = "SHOW transaction_read_only"
+            check_query = "SELECT pg_is_in_recovery()"
 
             self.poll_query_until(
                 query=check_query,
-                expected="off",
+                expected=False,
                 dbname=dbname,
                 username=username,
                 max_attempts=0)    # infinite