Fix flaky test exttab1 and pxf_fdw

The flaky case happens when select an external table with option
"fill missing fields". By gdb the qe, this value is not false
on QE sometimes. When ProcessCopyOptions, we use intVal(defel->arg)
to parse the boolean value, which is not correct. Using defGetBoolean
to replace it.
Also fix a pxf_fdw test case, which should set fill_missing_fields to
true explicitly.

cherry pick from: f154e5

src/backend/commands/copy.c

@@ -1417,7 +1417,7 @@ ProcessCopyOptions(CopyState cstate,
 				ereport(ERROR,
 						(errcode(ERRCODE_SYNTAX_ERROR),
 						 errmsg("conflicting or redundant options")));
-			cstate->fill_missing = intVal(defel->arg);
+			cstate->fill_missing = defGetBoolean(defel);
 		}
 		else if (strcmp(defel->defname, "newline") == 0)
 		{