According to https://www.freebsd.org/releases/ 12.1 is no longer
supported.
This also seemed to cause some problems in the most recent runs of the
Cirrus CI tasks.
Thus this commit updates the used version to 12.2
We used the BUILD_TESTING variable in order to indicate whether or not
tests shall be built. However this variable is used by e.g. CTest
already and there it had a different default value. Therefore we now
deprecate BUILD_TESTING and use the tests option instead.
This also fits better with our other options in terms of casing.