fix CMake warnings when Zoltan is enabled

[cz4rs]

gcc-9 build is getting this warning multiple times:

Call Stack (most recent call first):
  tests/CMakeLists.txt:84 (link_target_with_vt)
  tests/CMakeLists.txt:211 (add_unit_test)
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at cmake/link_vt.cmake:80 (target_link_libraries):
  The library that is being linked to, zoltan, is marked as being deprecated
  by the owner.  The message provided by the developer is:

  WARNING: The non-namespaced target 'zoltan' is deprecated! If always using
  newer versions of the project 'Trilinos', then use the new namespaced
  target 'Zoltan::zoltan', or better yet, 'Zoltan::all_libs' to be less
  sensitive to changes in the definition of targets in the package 'Zoltan'.
  Or, to maintain compatibility with older or newer versions the project
  'Trilinos', instead link against the libraries specified by the variable
  'Zoltan_LIBRARIES'.

Check which of the suggested solutions is better in our case and apply it.