tests: use GLib for library test cases and bats for gpio-tools

tests: use GLib for library test cases and bats for gpio-tools

This replaces the custom hand-crafted testing suite with well-known
open-source tools while keeping the same coverage. The core library
is now tested using the GLib unit testing framework. The gpio-tools
are now tested in bash using the bats testing system instead of being
invoked from a C program.

This simplifies and shrinks the testing code and makes it less prone
to bugs.

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