tests: drivers: stepper: stepper_api: Fix UAF of poll signal

The test_target_position test used a stack allocated `poll_signal` which is
kept as a refrence inside the gpio driver. Since the time for polling the
signal was chosen pretty narrow, the function exited with the driver still
trying to signal the stack allocated value upon movement completion.
Fix by adding the structs to the test fixture and increase the timeout.

Resolves #78466.

Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>