MDEV-19485: Add a test case

The bug was introduced in MariaDB 10.4.0 by
commit 0e5a4ac2
but it is good to have a regression test for this scenario
in all applicable MariaDB versions.

Cover the purge of an undo log record that was written before
the completion of ADD SPATIAL INDEX.

mysql-test/suite/innodb_gis/r/rtree_add_index.result

+SET @saved_frequency = @@GLOBAL.innodb_purge_rseg_truncate_frequency;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
+CREATE TABLE t1 (g MULTIPOINT NOT NULL) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ('');
+connect purge_control,localhost,root;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection default;
+DELETE FROM t1;
+ALTER TABLE t1 ADD SPATIAL INDEX (g);
+disconnect purge_control;
+InnoDB		0 transactions not purged
+DROP TABLE t1;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;

mysql-test/suite/innodb_gis/t/rtree_add_index.test

+--source include/have_innodb.inc
+
+SET @saved_frequency = @@GLOBAL.innodb_purge_rseg_truncate_frequency;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
+
+CREATE TABLE t1 (g MULTIPOINT NOT NULL) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ('');
+
+connect purge_control,localhost,root;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection default;
+
+DELETE FROM t1;
+
+ALTER TABLE t1 ADD SPATIAL INDEX (g);
+
+disconnect purge_control;
+--source ../../innodb/include/wait_all_purged.inc
+DROP TABLE t1;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;