There is currently no way of changing log level of TF via configuration. All logging seems to happen at INFO level which is not ideal when one needs to debug.
Trying to add a log4j config file in classpath and lowering level for info.magnolia.test at DEBUG, for instance, seems to be ignored.

Dev Notes

TF has actually a logging configuration file which I completely overlooked.
It is just a matter of enabling the proper log level there. Let's document this in TF docs so that we don't forget about it.