test: Improve coverage of several (member) functions

[stotko]

Some (member) functions are still not yet covered by the test suite. Address this issue by extending the unit tests of the memory, deque and vector modules.

[codecov]

Codecov Report

Merging #84 into master will increase coverage by 3.66%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   94.16%   97.82%   +3.66%     
==========================================
  Files          27       27              
  Lines        1610     1611       +1     
==========================================
+ Hits         1516     1576      +60     
+ Misses         94       35      -59

Impacted Files	Coverage Δ
src/stdgpu/impl/memory_detail.h	98.63% <0%> (+6.9%)	⬆️
src/stdgpu/impl/deque_detail.cuh	94% <0%> (+13.5%)	⬆️
src/stdgpu/impl/vector_detail.cuh	94.48% <0%> (+15.17%)	⬆️

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