TrfTestsARTPlots: try to switch off logfile buffering

This MR tries to switch off logfile buffering for the Derivation_tf part that is executed in AthenaMP for the TrfTestsARTPlots package.

Sometimes there are AthenaMP subworker crashes in these ART tests due to unknown reasons with e.g. exit code 65 (probably Grid related) and the last few lines of all subworker log files are missing. This is an attempt to unbuffer the log files for the Derivation_tf step since this is executed in AthenaMP+SharedWriter. I took inspiration from https://stackoverflow.com/questions/3465619/how-to-make-output-stdout-stderr-of-any-shell-command-unbuffered and I am not sure if this will actually address the problem but it will do no harm to these ART tests.

Tag @tsulaia for info.