add regression test for array

testcases/regression/invalid/Array1.scala

+object Array1 {
+
+  def foo(): Int = {
+    val a = Array.fill(5)(0)
+    a(2) = 3
+    a(2)
+  } ensuring(_ == 0)
+
+}

testcases/regression/invalid/Array2.scala

+object Array2 {
+
+  def foo(): Int = {
+    val a = Array.fill(5)(0)
+    a(2) = 3
+    a.length
+  } ensuring(_ == 4)
+
+}

testcases/regression/invalid/Array3.scala

+import leon.Utils._
+
+object Array3 {
+
+  def foo(): Int = {
+    val a = Array.fill(5)(3)
+    var i = 0
+    var sum = 0
+    (while(i <= a.length) {
+      sum = sum + a(i)
+      i = i + 1
+    }) invariant(i >= 0)
+    sum
+  } ensuring(_ == 15)
+
+}