[WIP] [stable6] Trying to fix failing encryption tests on Oracle

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