Fixed a minor glitch in each of sub_test and paratest.server

* In sub_test, eliminate the confusion between $CHPL_HOME/test/users/USERNAME/... and /users/USERNAME.

* In paratest.server --show-all-errors, show each error just once. While there, streamlined the code.

This looks ok, but you might add a comment to justify why the current directory is always the right one in this context.