They are supporting libraries and as such, will need to be built/installed when MK_TESTS == no during buildworld, i.e., the "make libraries" phase. Otherwise, dependent components cannot rely on the libraries, like `cddl/usr.sbin/zfsd/tests`.
I need to doublecheck my work vs the port, but I believe that this covers the initial integration of all upstream tests. Ref: https://github.com/ngie-eign/freebsd/tree/googletest-integration