Fix add_suppresion in partition_recover_myisam.

error message would contain full path to ha_myisam.cc,
if MySQL is compiled with CMake but suppression assumed
only the filename without path.

mysql-test/suite/parts/r/partition_recover_myisam.result

 call mtr.add_suppression("./test/t1_will_crash");
-call mtr.add_suppression("Got an error from unknown thread, ha_myisam.cc");
+call mtr.add_suppression("Got an error from unknown thread");
 CREATE TABLE t1_will_crash (a INT, KEY (a)) ENGINE=MyISAM;
 INSERT INTO t1_will_crash VALUES (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11);
 FLUSH TABLES;

mysql-test/suite/parts/t/partition_recover_myisam.test

 # test the auto-recover (--myisam-recover) of partitioned myisam tables
 
 call mtr.add_suppression("./test/t1_will_crash");
-call mtr.add_suppression("Got an error from unknown thread, ha_myisam.cc");
+call mtr.add_suppression("Got an error from unknown thread");
 
 --source include/have_partition.inc
 --disable_warnings