Gripper in tests with joint data.

[martiniil]

We need a uniform way of handling the gripper in tests. Currently we are circumventing the problem: https://github.com/PilzDE/pilz_industrial_motion/blob/6cce6beb3b35060d2b04340898e4c9dc4381a0c0/pilz_trajectory_generation/test/unittest_trajectory_generator_circ.cpp#L440 The following options could be considered:

1. All tests are run with gripper by default.
2. There are tests with and without gripper. Then joint test data should not be read from a file, but generated from cartesian test data in a uniform way.

[martiniil]

In this context, it should also be avoided to load two robot descriptions (one with and one without gripper) onto the parameter server. See https://github.com/PilzDE/pilz_robots/pull/94