Fix fmod(x, INFINITY)

[peterschrammel]

The case where the second argument of fmod is +-inf does not give the correct result with the current implementation in float_utils.rem.
I've added a regression test that does basic checks on the most important cases.

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[codecov]

Codecov Report

Merging #6414 (c4deb56) into develop (093a216) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6414   +/-   ##
========================================
  Coverage    75.98%   75.98%           
========================================
  Files         1524     1524           
  Lines       164242   164244    +2     
========================================
+ Hits        124801   124809    +8     
+ Misses       39441    39435    -6     

Impacted Files	Coverage Δ
src/solvers/floatbv/float_utils.cpp	95.71% <100.00%> (+0.01%)	⬆️
src/ansi-c/c_typecheck_expr.cpp	74.72% <0.00%> (+0.28%)	⬆️

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