bts: limit stderr logging to ERROR to avoid long write() to ext4 fs
Commit c3ab9ec33f37c6837fb8bcb0fa0bbd2e98781fb7 already decreased verbosity to NOTICE, but that's not enough since we still get plenty of logging and hence plenty of long write()s during test execution.
Let's try decreasing to ERROR and see if that's enough for now to stabilize the tests.
