test: add tests for fast_subsitute on function of array of symbolics

Author: AayushSabharwal

test/utils.jl

@@ -153,4 +153,12 @@ end
     test_nested_derivative = Dx(Dt(Dt(u)))
     result = diff2term(Symbolics.value(test_nested_derivative))
     @test typeof(result) === Symbolics.BasicSymbolic{Real}
-end
+end
+
+@testset "`fast_substitute` inside array symbolics" begin
+    @variables x y z
+    @register_symbolic foo(a::AbstractArray, b)
+    ex = foo([x, y], z)
+    ex2 = Symbolics.fixpoint_sub(ex, Dict(y => 1.0, z => 2.0))
+    @test isequal(ex2, foo([x, 1.0], 2.0))
+end