Fix a bug in sub_test.py that caused it to miss .prediff files if start_test was run from the directory the test was in, for example:
Fix a bug in sub_test.py that caused it to miss .prediff files if start_test
was run from the directory the test was in, for example:

cd test/release/examples/tutorials
$CHPL_HOME/util/start_test opaque.chpl


git-svn-id: 88467cb1fb04b8a755be7e1ee1026be4190196ef@16500 3a8e244f-b0f2-452b-bcba-4c88e055c3ca

hildeth/chapel,chizarlicious/chapel,hildeth/chapel,sungeunchoi/chapel,chizarlicious/chapel,CoryMcCartan/chapel,sungeunchoi/chapel,hildeth/chapel,chizarlicious/chapel,hildeth/chapel,CoryMcCartan/chapel,chizarlicious/chapel,CoryMcCartan/chapel,sungeunchoi/chapel,CoryMcCartan/chapel,sungeunchoi/chapel,sungeunchoi/chapel,CoryMcCartan/chapel,sungeunchoi/chapel,hildeth/chapel,CoryMcCartan/chapel,sungeunchoi/chapel,chizarlicious/chapel,CoryMcCartan/chapel,chizarlicious/chapel,hildeth/chapel,hildeth/chapel,sungeunchoi/chapel,chizarlicious/chapel