Add options to drop self-loops & multi_edges in C++ test graph generation

[seunghwak]

Necessary for #1909

Add options to filter out self-loops & multi-edges after edge generation and before graph creation in C++ test graph generation API.

[codecov-commenter]

Codecov Report

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@@               Coverage Diff               @@
##             branch-21.12    #1934   +/-   ##
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  Coverage                ?   70.04%           
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  Files                   ?      143           
  Lines                   ?     8904           
  Branches                ?        0           
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  Hits                    ?     6237           
  Misses                  ?     2667           
  Partials                ?        0           

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[BradReesWork]

rerun tests

[BradReesWork]

@gpucibot merge