Make the Chapel library copy command in util/test_python.bash more general

By using the environment variable $VIRTUAL_ENV instead of a hard-coded path, we
can more easily utilize the same script for our nightly testing and the Travis
build.
Merge pull request #2 from chapel-lang/generallyVirtual

Make the Chapel library copy command in util/test_python.bash more general

By using the environment variable $VIRTUAL_ENV instead of a hard-coded path, we
can more easily utilize the same script for our nightly testing and the Travis
build.