Add tests for default interface methods

Author: AayushSabharwal

test/default_function_test.jl

@@ -0,0 +1,10 @@
+using SymbolicIndexingInterface, Test
+
+@test independent_variables(nothing) == []
+@test states(nothing) == []
+@test parameters(nothing) == []
+@test !is_indep_sym(nothing, :a)
+@test !is_state_sym(nothing, :a)
+@test !is_param_sym(nothing, :a)
+@test isnothing(state_sym_to_index(nothing, :a))
+@test isnothing(param_sym_to_index(nothing, :a))

test/runtests.jl

@@ -2,3 +2,4 @@ using SymbolicIndexingInterface
 using Test
 
 @time begin @time @testset begin include("symbolcache.jl") end end
+@time begin @time @testset begin include("default_function_test.jl") end end