src.conf: Add a MK_ZFS_TESTS knob

The in-tree ZFS test suite is somewhat outdated and I see a number of
failures there. I tend to think that we want to integrate the OpenZFS
test suite somehow, replacing the legacy one, though it's also possible
to run that as a separate test suite.

In any case, if one wants to run the OpenZFS test suite separately, it's
useful to be able to disable installation of the legacy ZFS test suite,
so let's provide a src.conf option to do that.

Reviewed by: asomers
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D46476