Fix ImportRG Copy destination

The CMake command file(COPY) needs the destination for copying to be a
directory.  Change the TEST_VISTA_CACHE_DAT_VISTA filepath variable to be
the TEST_VISTA_CACHE_DIR_VISTA path variable.  Update the variable in the
ImportRG.cmake.in script.

Change-Id: I0ef40e521a4ba839073230e90424946c7c199ba9