Only build cugraphmgtestutil when requested

[robertmaynard]

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

[codecov-commenter]

Codecov Report

Merging #2454 (3db6078) into branch-22.08 (d925926) will increase coverage by 0.01%.
The diff coverage is 50.00%.

❗ Current head 3db6078 differs from pull request most recent head 1685b04. Consider uploading reports for the commit 1685b04 to get more accurate results

@@               Coverage Diff                @@
##           branch-22.08    #2454      +/-   ##
================================================
+ Coverage         61.25%   61.26%   +0.01%     
================================================
  Files               106      106              
  Lines              5400     5404       +4     
================================================
+ Hits               3308     3311       +3     
- Misses             2092     2093       +1     

Impacted Files	Coverage Δ
...ugraph/cugraph/dask/structure/mg_property_graph.py	17.50% <0.00%> (-0.13%)	⬇️
python/cugraph/cugraph/structure/property_graph.py	96.20% <100.00%> (+0.02%)	⬆️
...ython/cugraph/cugraph/community/ktruss_subgraph.py	88.23% <0.00%> (+2.94%)	⬆️

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[ChuckHastings]

@gpucibot merge