The test innodb_zip.index_large_prefix_4k would not run unless it is
invoked as
./mtr --mysqld=--innodb-page-size=4k innodb_zip.index_large_prefix_4k

This test was originally developed to cover an option that was removed
in commit 0c92794d. Starting with
MariaDB Server 10.2, which introduced innodb_default_row_format=dynamic,
the option innodb_large_prefix had become useless.

Let us remove some of the stale tests and adjust the outcome to the
expected behaviour.