[#3234] Change r_print to var_export for diff generation of array subsets

[1blankz7]

Resolves #3234

[codecov-io]

Codecov Report

Merging #3283 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #3283   +/-   ##
=========================================
  Coverage     82.36%   82.36%           
  Complexity     3486     3486           
=========================================
  Files           140      140           
  Lines          9210     9210           
=========================================
  Hits           7586     7586           
  Misses         1624     1624

Impacted Files	Coverage Δ	Complexity Δ
src/Framework/Constraint/ArraySubset.php	96.87% <100%> (ø)	11 <0> (ø)	⬇️

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[sebastianbergmann]

Merged manually into 6.5 and merged to 7.3 and master from there.