[test] H5: check ascii in file to utf8 memory

[gicmo]

Check that ASCII encoded data in the file can be read into memory.

[codecov[bot]]

Codecov Report

Merging #817 (535c07b) into master (c865fd0) will decrease coverage by 0.00%. The diff coverage is 92.50%.

@@            Coverage Diff             @@
##           master     #817      +/-   ##
==========================================
- Coverage   95.35%   95.34%   -0.01%     
==========================================
  Files         181      181              
  Lines       13558    13598      +40     
==========================================
+ Hits        12928    12965      +37     
- Misses        630      633       +3     

Impacted Files	Coverage Δ
backend/hdf5/h5x/H5DataType.cpp	81.78% <90.62%> (+1.31%)	:arrow_up:
test/hdf5/TestH5.cpp	100.00% <100.00%> (ø)

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