MDEV-26554: Stabilize the test

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

@@ -919,6 +919,7 @@ ERROR HY000: Lock wait timeout exceeded; try restarting transaction
 ALTER TABLE parent ADD COLUMN b INT, ALGORITHM=INSTANT;
 ERROR HY000: Lock wait timeout exceeded; try restarting transaction
 disconnect con1;
+SET innodb_lock_wait_timeout=DEFAULT;
 TRUNCATE TABLE parent;
 ALTER TABLE parent FORCE, ALGORITHM=COPY;
 ALTER TABLE parent FORCE, ALGORITHM=INPLACE;

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

@@ -925,6 +925,8 @@ ALTER TABLE parent FORCE, ALGORITHM=INPLACE;
 --error ER_LOCK_WAIT_TIMEOUT
 ALTER TABLE parent ADD COLUMN b INT, ALGORITHM=INSTANT;
 disconnect con1;
+# Restore the timeout to avoid occasional races with purge.
+SET innodb_lock_wait_timeout=DEFAULT;
 TRUNCATE TABLE parent;
 ALTER TABLE parent FORCE, ALGORITHM=COPY;
 ALTER TABLE parent FORCE, ALGORITHM=INPLACE;