MyISAM allows CHECK constraint violation in ALTER TABLE

The test case added in
MDEV-12389 ADD CHECK leaves an orphaned .par file
fails in an unexpected way in 10.2.
For InnoDB, the statement

ALTER TABLE t1 ADD CHECK(b IN (0,1));

fails as expected, because the column contains the values 0 and 10. However, MyISAM accepts the invalid constraint and only enforces it on subsequent DML operations.