Tests should no longer use sleep_for

[ddengster]

See #25 (comment)

• Find a way to wait for services
• Apply the change over all tests

[bmagyar]

@v-lopez do you have an idea for this?

[v-lopez]

I am still at the beginning of the learning process with all this. So not at the moment.
But I can take a look in a couple of weeks when I'm more familiar with ROS2 and ros2_control.

But I totally agree that random sleeps in tests are recipe for flakiness.

[bmagyar]

@jaron-l this may be relevant for your tickets