Require CMocka >= 1.1.3 to run the unit tests
In CMocka versions << 1.1.3, the skip() function would cause the whole unit test to abort when CMOCKA_TEST_ABORT is set. As this is problem only in Debian 9 Stretch and Ubuntu 16.04 Xenial, we just require the CMocka >= 1.1.3 and disable the unit testing on Debian 9 Stretch until we can pull the libcmocka-dev from stretch-backports and remove the Ubuntu 16.04 Xenial from the CI as it is reaching End of Standard Support at the end of April 2021.