Free ignorelist after each regression test schedule.

It's a trivial amount of RAM held until the end of the regression
test run; but it's probably worth fixing to silence future warnings
from code analyzers.

This was the only memory leak pointed out by clang's static code
analysis tool.

src/test/regress/pg_regress.c

@@ -1655,6 +1655,8 @@ run_schedule(const char *schedule, test_function tfunc)
 		}
 	}
 
+	free_stringlist(&ignorelist);
+
 	fclose(scf);
 }