fix parallel_gripper_controller failing tests

[saikishor]

The Parallel gripper controller tests are not passing on the rolling from beginning, I've modified the tests to make it work. However, I had to remove a couple of tests as in the GPL the parameter joint is declared as read_only, and this makes most of the tests doesn't work at all, so I've cleaned them up

ros2_controllers/parallel_gripper_controller/src/gripper_action_controller_parameters.yaml

Lines 11 to 14 in 0bb8880

	joint: {
	type: string,
	read_only: true,
	}

[christophfroehlich]

Should we use GPL for validation instead of checking params_.joint.empty()? Then there is no need for testing it (see the deleted tests).

[destogl]

Agree with @christophfroehlich, let's use GenerateParameterLiberry validators instead of testing empty joints.

[christophfroehlich]

Thanks for the changes (haven't tested it with #1206, but I guess you did?)

[destogl]

Thanks!