[WIP] Stable6 fix trial for failing oracle encryption tests

In `apps/files_encryption/lib/util.php` in `beginMigration` an update SQL operation is made on the DB. That update expects the query to only return a single row (`manipulatedRows`), but sometimes we got 2 or 4 rows, which seem to mean that there is old data still available in the DB.

To check for this, this PR has a commit that explicitly deletes the contents of the `oc_encryption` table in `setUp` and `tearDown` to make sure we always start with a clean table.
But from here, we still get more than one row. :frowning: 

Maybe there is a cleanup that is supposed to happen in the middle of tests or something else.
Needs further research.

CC @schiesbn @DeepDiver1975 @butonic @karlitschek