Add initial_value checks to state interface tests (backport #765)

[mergify]

This PR adds a test to verify that the slider_to_cart joint is present in
/joint_states before the velocity example starts moving the robot.

This addresses the part of issue #764 related to checking initial_value
availability before motion. The test does not assert the numeric value of the
initial position, since /joint_states may publish an uninitialized value in
the first cycles of simulation. Instead, it ensures that the joint is present
and published before the controller starts sending commands.

All tests pass locally, including flake8.

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This is an automatic backport of pull request #765 done by [Mergify](https://mergify.com).