MDEV-15326 preparation: Test slow shutdown after XA PREPARE

We were missing a test that would exercise trx_free_prepared()
with innodb_fast_shutdown=0. Add a test.

Note: if shutdown hangs due to the XA PREPARE transactions,
in MariaDB 10.2 the test would unfortunately pass, but take
2*60 seconds longer, because of two shutdown_server statements
timing out after 60 seconds. Starting with MariaDB 10.3, the
hung server would be killed with SIGABRT, and the test could
fail thanks to a backtrace message.

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

+FLUSH TABLES;
+call mtr.add_suppression("Found 1 prepared XA transactions");
 #
 # MDEV-13797 InnoDB may hang if shutdown is initiated soon after startup
 # while rolling back recovered incomplete transactions
@@ -7,10 +9,12 @@ BEGIN;
 COMMIT;
 connect  con$c,localhost,root,,;
 CREATE TABLE t8 (a SERIAL, b INT UNIQUE, c INT UNIQUE) ENGINE=InnoDB;
-BEGIN;
+XA START 'x';
 INSERT INTO t8 (a) SELECT NULL FROM t;
 UPDATE t8 SET a=a+100, b=a;
 DELETE FROM t8;
+XA END 'x';
+XA PREPARE 'x';
 connect  con$c,localhost,root,,;
 CREATE TABLE t7 (a SERIAL, b INT UNIQUE, c INT UNIQUE) ENGINE=InnoDB;
 BEGIN;
@@ -62,4 +66,7 @@ connection default;
 SET GLOBAL innodb_flush_log_at_trx_commit=1;
 CREATE TABLE u(a SERIAL) ENGINE=INNODB;
 FLUSH TABLES;
+XA RECOVER;
+formatID	gtrid_length	bqual_length	data
+1	1	0	x
 DROP TABLE t,u;

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

 --source include/have_innodb.inc
 --source include/not_embedded.inc
 
+# Flush any open myisam tables from previous tests
+FLUSH TABLES;
+call mtr.add_suppression("Found 1 prepared XA transactions");
+
 --echo #
 --echo # MDEV-13797 InnoDB may hang if shutdown is initiated soon after startup
 --echo # while rolling back recovered incomplete transactions
@@ -20,6 +24,15 @@ dec $c;
 COMMIT;
 
 let $c = $trx;
+connect (con$c,localhost,root,,);
+eval CREATE TABLE t$c (a SERIAL, b INT UNIQUE, c INT UNIQUE) ENGINE=InnoDB;
+XA START 'x';
+eval INSERT INTO t$c (a) SELECT NULL FROM t;
+eval UPDATE t$c SET a=a+$size, b=a;
+eval DELETE FROM t$c;
+XA END 'x';
+XA PREPARE 'x';
+dec $c;
 while ($c)
 {
 connect (con$c,localhost,root,,);
@@ -50,12 +63,17 @@ FLUSH TABLES;
 
 # Perform a slow shutdown in order to roll back all recovered transactions
 # and to avoid locking conflicts with the DROP TABLE below.
+XA RECOVER;
 --disable_query_log
 SET GLOBAL innodb_fast_shutdown=0;
 --source include/restart_mysqld.inc
 
 --disable_query_log
 let $c = $trx;
+disconnect con$c;
+XA ROLLBACK 'x';
+eval DROP TABLE t$c;
+dec $c;
 while ($c)
 {
 disconnect con$c;