Bugfix importing tools in RunTier0Test.py

In !20514 (merged) we started importing a new module in RunTier0Test.py. When running locally (also on the CI) everything is fine since the new module is in the same directory and the executable for RunTier0Test.py is simply a symlink to the file in that folder. However, this doesn't fly, obviously, in the nightly as the executable is copied under the appropriate bin/ folder so the new module cannot be found. This MR fixes that issue.