Fix some crashes in ALTER TABLE…IMPORT TABLESPACE

MySQL 5.7.29 includes a fix for avoiding some crashes in IndexPurge::next() in ALTER TABLE…IMPORT TABLESPACE:
Bug#29454828 ASSERTION ON IMPORT VIA TRANSPORTABLE TABLESPACE
The test case introduces some fault injection that does not appear to cover the changed functionality.

MariaDB should fix this in a better way, by replacing the btr_pcur_move_to_next_user_rec() call with equivalent code, and replacing the btr_pcur_move_to_next_page() call with code that will not kill the server, but return an error to the caller.
Ultimately, in MDEV-13542 we should make btr_pcur_move_to_next_user_rec() robust, and check the status in all callers.