Fix a warn that should be a @warn in tests

This warning hasn't been triggered in a while, so this has slipped through.

test/runtests.jl

@@ -711,7 +711,7 @@ end
         run!(runner, y, u)
         alloc = @allocated run!(runner, y, u)
         if alloc > 0
-            warn("Allocated $alloc in run! of HomotopySolver{SimpleSolver} base ModelRunner")
+            @warn "Allocated $alloc in run! of HomotopySolver{SimpleSolver} base ModelRunner"
         end
     end