Refine-arrays: work with refine-strings

[smowton]

Trivial fix to allow running the string solver with --refine-arrays. Doesn't seem to help much when the index set is large, but at least this means we don't ignore --refine-strings when --refine-arrays is passed.

[codecov-io]

Codecov Report

Merging #5245 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #5245      +/-   ##
===========================================
+ Coverage    67.43%   67.43%   +<.01%     
===========================================
  Files         1162     1162              
  Lines        95620    95622       +2     
===========================================
+ Hits         64477    64479       +2     
  Misses       31143    31143

Flag	Coverage Δ
#cproversmt2	42.59% <100%> (ø)	⬆️
#regression	63.96% <100%> (ø)	⬆️
#unit	31.89% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/goto-checker/solver_factory.cpp	69.28% <100%> (+0.4%)	⬆️

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