Existing tests correction: forbid val reassignment

compiler/testData/codegen/classes/overloadPlusAssign.jet

@@ -17,7 +17,7 @@ class ArrayWrapper<T>() {
 }
 
 fun box(): String {
-    val v1 = ArrayWrapper<String>()
+    var v1 = ArrayWrapper<String>()
     val v2 = ArrayWrapper<String>()
     v1.add("foo")
     v2.add("bar")

compiler/testData/codegen/classes/overloadPlusAssignReturn.jet

@@ -20,7 +20,7 @@ class ArrayWrapper<T>() {
 }
 
 fun box(): String {
-    val v1 = ArrayWrapper<String>()
+    var v1 = ArrayWrapper<String>()
     val v2 = ArrayWrapper<String>()
     v1.add("foo")
     val v3 = v1

compiler/testData/codegen/classes/overloadPlusToPlusAssign.jet

@@ -20,7 +20,7 @@ class ArrayWrapper<T>() {
 }
 
 fun box(): String {
-    val v1 = ArrayWrapper<String>()
+    var v1 = ArrayWrapper<String>()
     val v2 = ArrayWrapper<String>()
     v1.add("foo")
     val v3 = v1

compiler/testData/codegen/controlStructures/forIntRange.jet

@@ -4,7 +4,7 @@ fun box() : String {
     a[1] = "b"
     a[2] = "c"
 
-    val result = 0
+    var result = 0
     for(i in a.indices) {
       result += i
     }