tests: mockup: validate the chip index where applicable

tests: mockup: validate the chip index where applicable

In functions that take the chip index as argument: let's verify that
its value is valid i.e. it refers to an existing chip to avoid potential
segfaults.

While we're at it: delegate the validation of arguments in mockup getter
helpers to a separate routine.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>