test: add actual test around the gradient

Co-authored-by: Aayush Sabharwal <aayush.sabharwal@gmail.com>

Author: SebastianM-C

lib/ModelingToolkitBase/test/initializationsystem.jl

@@ -2066,5 +2066,5 @@ end
         sum(abs2, newsol[sys.s])
     end
 
-    ForwardDiff.gradient(loss, [3.0, 20.0])
+    @test_nowarn ForwardDiff.gradient(loss, [3.0, 20.0])
 end