pytest: set dblog-file when adding the dblog plugin (TEST_CHECK_DBSTMTS=1)

As we'll see in the next patch, this wasn't *supposed* to work wihtout dblog-file,
but it did, creating a dblog called "null".

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/fixtures.py

@@ -40,6 +40,7 @@ class LightningNode(utils.LightningNode):
             if not has_dblog:
                 # Add as an expanded option so we don't clobber other options.
                 self.daemon.opts['plugin={}'.format(dblog)] = None
+                self.daemon.opts['dblog-file'] = 'dblog.sqlite3'
 
         # Yes, we really want to test the local development version, not
         # something in out path.