bindings: python: use unittest to implement a proper test-suite

bindings: python: use unittest to implement a proper test-suite

Currently the only tests we have for python bindings are in a simple
script that makes a lot of assumptions about the environment and
doesn't even work anymore with recent kernels.

Remove it and replace it with a proper test-suite implemented using
libgpiod and the standard python unittest package.

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