Specify file-based sqlite db in tox

tox.ini

@@ -61,6 +61,8 @@ setenv =
     mysql: DATABASE_HOST=localhost
     mysql: DATABASE_USER=root
 
+    sqlite: DATABASE_NAME=wagtail.db
+
     # Specific for Appveyor, see:
     # https://www.appveyor.com/docs/services-databases/#sql-server-2016
     mssql: DATABASE_ENGINE=sql_server.pyodbc