[AutoDiff] Add tests exercising zero derivative of Array.DifferentiableView.+ and .move

Tests for false positive precondition triggered by a .zero gradient, i.e. empty [] array, back-propagated through concatenation and move methods
Implements a simplified reproducer of SR-14297

Author: vojtamolda

test/AutoDiff/validation-test/array.swift

@@ -344,6 +344,10 @@ ArrayAutoDiffTests.test("Array.+") {
   }
   let v = FloatArrayTan([4, -5, 6])
   expectEqual(v, pullback(at: [1, 2, 3], of: identity)(v))
+  
+  let v1: [Float] = [1, 1]
+  let v2: [Float] = [1, 1, 1]
+  expectEqual((.zero, .zero), pullback(at: v1, v2, of: +)(.zero))
 }
 
 ArrayAutoDiffTests.test("Array.+=") {
@@ -438,4 +442,14 @@ ArrayAutoDiffTests.test("Array.DifferentiableView.base") {
     backprop(FloatArrayTan([1, 2, 3, 4])))
 }
 
+ArrayAutoDiffTests.test("Array.DifferentiableView.move") {
+  var v: [Float] = [1, 2, 3]
+  v.move(by: .zero)
+  expectEqual(v, [1, 2, 3])
+
+  var z: [Float] = []
+  z.move(by: .zero)
+  expectEqual(z, [])
+}
+
 runAllTests()