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.