statetest: Fix loading difficulty for BlockInfo

[rodiazet]

Following Spencer's fix added in #602

[codecov]

Codecov Report

Merging #603 (293edab) into master (fd99568) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 293edab differs from pull request most recent head 898ea5c. Consider uploading reports for the commit 898ea5c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #603      +/-   ##
==========================================
+ Coverage   97.07%   97.09%   +0.02%     
==========================================
  Files          74       74              
  Lines        7375     7397      +22     
==========================================
+ Hits         7159     7182      +23     
+ Misses        216      215       -1     

Flag	Coverage Δ
blockchaintests	63.71% <ø> (ø)
statetests	63.15% <71.42%> (+0.03%)	⬆️
unittests	93.93% <100.00%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
test/statetest/statetest_loader.cpp	88.46% <100.00%> (+0.59%)	⬆️
...est/unittests/statetest_loader_block_info_test.cpp	100.00% <100.00%> (ø)