DROP DATABASE would internally execute DROP TABLE on every contained
table and finally remove the directory. In InnoDB, DROP TABLE is
sometimes executed in the background. The table would be renamed to
a name that starts with #sql. The existence of these files would
prevent DROP DATABASE from succeeding.

CREATE OR REPLACE DATABASE can internally execute DROP DATABASE if
the directory already exists. This could fail due to the InnoDB
background DROP TABLE, possibly due to some tables that were
leftovers from earlier tests.