[Jenkins] Frequent timeout in tp_process_imt

See e.g.

http://cdash.cern.ch/viewTest.php?onlyfailed&buildid=518933

http://cdash.cern.ch/viewTest.php?onlyfailed&buildid=518952

http://cdash.cern.ch/viewTest.php?onlyfailed&buildid=518942

from the last round of nightlies.

The timeouts are most likely due to issues in reading files or parts of files from root.cern.ch.
As tp_process_imt is testing TTreeProcessorMT and not our network infrastructure, it was decided to reduce the size of the test file and move it inside roottest.

As long as the file contains a large number of clusters, the desired code-paths are tested.