It seems include_str! is evaluated at compile time for the per-problem 'solve()' functions, and the compilation fails of the referenced input files don't exist. (I would have naively expected compilation and test execution to work fine here, and for a runtime error to be reserved for when the function was actually called).

This is fine for me but will hinder the ability of someone else to pull the code and easily run the tests. I should refactor this or else just commit an input file for each problem.