Fix variable name and change tests to use MemorySource

[elshize]

No description provided.

[codecov]

Codecov Report

Merging #420 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage   92.42%   92.48%   +0.06%     
==========================================
  Files          91       91              
  Lines        4921     4925       +4     
==========================================
+ Hits         4548     4555       +7     
+ Misses        373      370       -3     

Impacted Files	Coverage Δ
include/pisa/wand_data.hpp	62.50% <ø> (ø)
include/pisa/block_freq_index.hpp	96.61% <100.00%> (+0.11%)	⬆️
include/pisa/freq_index.hpp	98.38% <100.00%> (+0.05%)	⬆️
include/pisa/mappable/mapper.hpp	85.71% <0.00%> (+3.29%)	⬆️

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