Reduce verbosity of hlt1_velo_decoding test

The hlt1_velo_decoding test configures an instance of TestVeloClusters, which prints a lot of INFO messages for every event. This results in ~100 MB of stdout, in turn causing some timeouts in the updated nightly dashboard.

Since these messages are not actually checked in the test validator, they are here removed by setting the OutputLevel to WARNING until a better fix is in place (e.g. fewer events or lower verbosity).