unordered_map,unordered_set: Extend unit tests

[stotko]

Although both unordered_map and unordered_set have probably the most extensive unit test suite, their code coverage is rather low. This is partially caused by the plethora of "read" operations to obtain certain kinds of information regarding their state. Extend the test suite to cover more of their operations and to increase the coverage.

[codecov]

Codecov Report

Merging #77 into master will increase coverage by 5%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #77   +/-   ##
=======================================
+ Coverage   81.77%   86.77%   +5%     
=======================================
  Files          27       27           
  Lines        1580     1580           
=======================================
+ Hits         1292     1371   +79     
+ Misses        288      209   -79

Impacted Files	Coverage Δ
src/stdgpu/impl/unordered_base_detail.cuh	88.83% <0%> (+9.89%)	⬆️
src/stdgpu/impl/unordered_map_detail.cuh	65.55% <0%> (+22.22%)	⬆️
src/stdgpu/impl/unordered_set_detail.cuh	64.77% <0%> (+22.72%)	⬆️

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