ranges: Add unit tests

[stotko]

The ranges module provides lightweight wrappers around a pair of iterators. In comparison to the iterator module, it lacks unit tests. Add he respective unit tests to improve the test coverage of the ranges module.

[codecov]

Codecov Report

Merging #76 into master will increase coverage by 0.56%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #76      +/-   ##
=========================================
+ Coverage   81.13%   81.7%   +0.56%     
=========================================
  Files          27      27              
  Lines        1580    1580              
=========================================
+ Hits         1282    1291       +9     
+ Misses        298     289       -9

Impacted Files	Coverage Δ
src/stdgpu/impl/unordered_base_detail.cuh	78.68% <0%> (-0.26%)	⬇️
src/stdgpu/impl/ranges_detail.h	100% <0%> (+38.46%)	⬆️

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