PyJobTransforms: Fix test.

Execute test_HelloWorldTrf in a subdirectory.  Otherwise, files it
needs (such as runargs.athena.py) can be clobbered by other
concurrently executing tests.

Tools/PyJobTransforms/test/test_HelloWorldTrf.sh

@@ -2,6 +2,9 @@
 
 set -e
 
+mkdir -p test_HelloWorldTrf
+cd test_HelloWorldTrf
+
 # ST test
 HelloWorld_tf.py --maxEvents=5 --CA