tests: do not use basic_string_view comparison as a workaround

[igchor]

for C2593: 'operator ==' is ambigous on Visual Studio 14.0

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This change is 

[lukaszstolarczuk]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @igchor)

[KFilipek]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @igchor)

[codecov]

Codecov Report

Merging #1126 (07e0764) into master (2a8e855) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1126      +/-   ##
==========================================
+ Coverage   94.21%   94.23%   +0.01%     
==========================================
  Files          52       52              
  Lines        5325     5325              
==========================================
+ Hits         5017     5018       +1     
+ Misses        308      307       -1     

Flag	Coverage Δ
tests_clang_debug_cpp17	93.77% <ø> (+0.07%)	⬆️
tests_gcc_debug	90.87% <ø> (-0.13%)	⬇️

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Impacted Files	Coverage Δ
...libpmemobj++/detail/enumerable_thread_specific.hpp	98.91% <0.00%> (-1.09%)	⬇️
include/libpmemobj++/container/vector.hpp	92.61% <0.00%> (ø)
include/libpmemobj++/experimental/radix_tree.hpp	98.43% <0.00%> (+0.14%)	⬆️
...ude/libpmemobj++/container/concurrent_hash_map.hpp	94.20% <0.00%> (+0.15%)	⬆️

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