Add unit test for add_builtin

[LucasSte]

Problem

#547 moved add_builtin to the SVM crate, but did not include a test for it.

Summary of Changes

I wrote a test for the aforementioned function.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.9%. Comparing base (72ee270) to head (30ced70).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #720    +/-   ##
========================================
  Coverage    81.9%    81.9%            
========================================
  Files         851      851            
  Lines      230726   230772    +46     
========================================
+ Hits       189009   189131   +122     
+ Misses      41717    41641    -76