tests: split out reusable test code into a local static library

tests: split out reusable test code into a local static library

In order to allow the upcoming GLib and DBus bindings to reuse the test
code, let's put all common elements into reusable libtool objects and
export the relevant symbols in internal headers.

Tested-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Link: https://lore.kernel.org/r/20240812-dbus-v5-1-ead288509217@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>