The issue was a bad merge of MDEV-12253 from 10.1 to 10.2
in commit f9cc3918.
In that merge, I wrongly assumed that all test file conflicts
for mysql-test/suite/encryption had been properly resolved in
bb-10.2-MDEV-12253 (commit 76aa6be77635c7017459ce33b41c837c9acb606d)
while in fact, some files there had been copied from the 10.1 branch.

This commit is based on a manually done conflict resolution of
the mysql-test/suite/encryption on the same merge, applied to
the current 10.2 branch.

As part of this commit, the test encryption.innodb-bad-key-change4
which was shortly disabled due to MDEV-11336 will be re-enabled again.
(While the test enables innodb_defragment, it does not fail even though
enabling innodb_defragment currently has no effect.)