MDEV-20934: Make the test more robust

Due to MDEV-12288, the slow shutdown in MariaDB 10.3 will include
resetting the DB_TRX_ID for all inserted records. This might
cause the 60-second shutdown_server timeout to be exceeded.
Let us wait for the purge to complete before initiating slow shutdown.

mysql-test/suite/innodb/r/ibuf_not_empty.result

@@ -22,5 +22,7 @@ check table t1;
 Table	Op	Msg_type	Msg_text
 test.t1	check	Warning	InnoDB: Index 'b' contains #### entries, should be 4096.
 test.t1	check	error	Corrupt
+SET GLOBAL innodb_purge_rseg_truncate_frequency=1;
+InnoDB		0 transactions not purged
 SET GLOBAL innodb_fast_shutdown=0;
 DROP TABLE t1;

mysql-test/suite/innodb/t/ibuf_not_empty.test

@@ -77,6 +77,12 @@ check table t1;
 
 --let $restart_parameters=
 --source include/restart_mysqld.inc
+
+# Ensure that the slow shutdown will not time out due to running purge.
+SET GLOBAL innodb_purge_rseg_truncate_frequency=1;
+--source include/wait_all_purged.inc
+# The change buffer merge for the injected corruption must complete
+# without exceeding the 60-second shutdown_server timeout.
 SET GLOBAL innodb_fast_shutdown=0;
 --source include/restart_mysqld.inc