Fix AbstractDiffEqArray getindex deprecation warnings in tests

Use `.u` to access underlying arrays from DiffEqArray objects returned
by solution interpolation before comparison with `≈`, instead of
relying on the deprecated `Base.getindex(::AbstractDiffEqArray, ::Int)`
method that gets triggered by broadcasting during `isapprox`.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: claude

test/integrators/iterator.jl

@@ -76,7 +76,7 @@ end
     for (u, t) in TimeChoiceIterator(integrator4, ts)
         push!(us, u)
     end
-    @test us ≈ integrator4.sol(ts)
+    @test us ≈ integrator4.sol(ts).u
 end
 
 @testset "iter" begin

test/interface/save_idxs.jl

@@ -42,7 +42,7 @@ const alg = MethodOfSteps(BS3())
             @test sol.u == dde_int.sol.u
 
             ## interpolation
-            @test sol(25:100, idxs = 2) ≈ [u[1] for u in sol(25:100, idxs = [2])]
+            @test sol(25:100, idxs = 2).u ≈ [u[1] for u in sol(25:100, idxs = [2]).u]
         end
 
         # with keyword argument
@@ -61,7 +61,7 @@ const alg = MethodOfSteps(BS3())
             @test sol.u == dde_int.sol.u
 
             # interpolation
-            @test sol[2, :] ≈ dde_int.integrator.sol(sol.t, idxs = 2)
+            @test sol[2, :] ≈ dde_int.integrator.sol(sol.t, idxs = 2).u
         end
     end
 
@@ -84,8 +84,8 @@ const alg = MethodOfSteps(BS3())
             @test sol2.u == dde_int2.sol.u
 
             # interpolation
-            @test sol(25:100, idxs = 2) ≈ sol2(25:100, idxs = 1)
-            @test sol(25:100, idxs = [2]) ≈ sol2(25:100, idxs = [1])
+            @test sol(25:100, idxs = 2).u ≈ sol2(25:100, idxs = 1).u
+            @test sol(25:100, idxs = [2]).u ≈ sol2(25:100, idxs = [1]).u
         end
 
         # scalar index
@@ -109,7 +109,7 @@ const alg = MethodOfSteps(BS3())
 
             # interpolation of solution equals second component of
             # interpolation of complete solution
-            @test sol(25:100, idxs = 2) ≈ sol2(25:100, idxs = 1)
+            @test sol(25:100, idxs = 2).u ≈ sol2(25:100, idxs = 1).u
         end
     end
 end