Update C++ MG PageRank and SG PageRank, Katz Centrality, BFS, and SSSP to use the new R-mat graph generator

[seunghwak]

• Refactor cuGraph C++ test library
• Add a utility function to create a graph object from the R-mat generator
• Update C++ MG PageRank and SG PageRank, Katz Centrality, BFS, and SSSP tests to use the new R-mat graph generator

This partially addresses #1382 and is a per-requsite for graph primitives performance optimization.

[codecov-io]

Codecov Report

Merging #1438 (1e25a4d) into branch-0.19 (369beee) will increase coverage by 0.05%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.19    #1438      +/-   ##
===============================================
+ Coverage        60.72%   60.78%   +0.05%     
===============================================
  Files               70       70              
  Lines             3132     3134       +2     
===============================================
+ Hits              1902     1905       +3     
+ Misses            1230     1229       -1     

Impacted Files	Coverage Δ
python/cugraph/_version.py	44.80% <0.00%> (+0.39%)	⬆️
python/cugraph/utilities/utils.py	72.44% <0.00%> (+0.78%)	⬆️

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[BradReesWork]

@gpucibot merge