tests: Run FuzzTest within a transaction

tests: Run FuzzTest within a transaction

Currently, the FuzzTests fail for me with:

/backends/base/base.py", line 428, in validate_no_broken_transaction
    "An error occurred in the current transaction. You can't "
TransactionManagementError: An error occurred in the current transaction. You can't execute queries until the end of the 'atomic' block.

- because the SQL inserts can fail, during an active transaction (the
first failure I see is attempting to insert \0 chars in
codec-null.mbox); this causes the setup for the next test case to fail.

Instead, run each test in its own transaction.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Reviewed-by: Stephen Finucane <stephen@that.guru>