Mk/Uses/cmake.mk: Add CMAKE_TESTING_SETENV

Defining this variable in an individual port Makefile sets the
environment for building and, more importantly, running the target
defined by CMAKE_TESTING_TARGET with SETENV instead of SETENVI.

Certain tests may require a running X or Wayland session to function
and SETENVI discards important environment variables like DISPLAY and
XAUTHORITY that cause these tests to fail or segfault.