add explicit check for virtualenv in the test

Author: dhellmann

tests/test_mkvirtualenv.sh

@@ -56,6 +56,7 @@ GLOBAL postmkvirtualenv"
 test_no_virtualenv () {
     old_path="$PATH"
     PATH="/usr/bin:/bin:/usr/sbin:/sbin"
+    assertFalse "Found virtualenv in $(which virtualenv)" "which virtualenv"
     mkvirtualenv should_not_be_created 2>/dev/null
     RC=$?
     # Restore the path before testing because