hw: add missing test_depend on description pkg. Fix #5.

`franka_hw` test depends on `franka_description`, but the pkg manifest did
not declare this dependency, leading to build ordering problems with
`franka_hw` being built in parallel to `franka_description`.

As `hw` depends on `description`, make the test depend explicit.

franka_hw/package.xml

@@ -19,6 +19,7 @@
   <depend>joint_limits_interface</depend>
   <depend>roscpp</depend>
 
+  <test_depend>franka_description</test_depend>
   <test_depend>gtest</test_depend>
   <test_depend>rostest</test_depend>
 </package>