bindings: cxx: new test cases
bindings: cxx: new test cases

Implement more test cases for C++ bindings and extend the existing
ones.

While we're at it: use auto where applicable and log to stderr instead
of stdout.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

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