README: update the testing section
README: update the testing section

Add information about the tests included in Python and C++ bindings.
Move the BINDINGS section above TESTING so that the user first knows
that bindings exist before learning they also provide their own
test-suites.

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

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