Only build cugraphmgtestutil when requested (#2454)

Guard the compilation of cugraphmgtestutil to only happen when BUILD_CUGRAPH_MG_TESTS is enabled so you can build a subset of tests without MPI

Authors:
  - Robert Maynard (https://github.com/robertmaynard)

Approvers:
  - Chuck Hastings (https://github.com/ChuckHastings)
  - Rick Ratzel (https://github.com/rlratzel)

URL: #2454