Update SG PageRank C++ tests

[seunghwak]

• Add const to input pointers.
• Use a double type counter in std::accumulate as std::accumulate is inaccurate in adding a large number of a comparably sized values.
• Fix random number generator seed.
• Re-enable a temporarily disabled test case.
• Relax the thresholds to skip comparison for lowly ranked vertices (with low scores which are more susceptible to the limited floating-point resolution)

[codecov-io]

Codecov Report

Merging #1307 (ca89bde) into branch-0.18 (049b088) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           branch-0.18    #1307   +/-   ##
============================================
  Coverage        60.21%   60.21%           
============================================
  Files               67       67           
  Lines             3049     3049           
============================================
  Hits              1836     1836           
  Misses            1213     1213           

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