tests: fix debugfs path
tests: fix debugfs path

Since linux v5.1 the debugfs path for gpio-mockup changed. Use the new
path both for the main directory as well as for the chip subdirectories
which no longer use the chip's label.

Signed-off-by: Bartosz Golaszewski <e5d45fddd9040473413b909b802f629a971ad04c@baylibre.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod