backend: add tests for lambda functions

backend.native/tests/codegen/lambda/lambda4.kt

@@ -0,0 +1,28 @@
+fun main(args : Array<String>) {
+    val lambda = bar()
+    lambda()
+    lambda()
+}
+
+fun bar(): () -> Unit {
+    var x = Integer(0)
+
+    val lambda = {
+        println(x.toString())
+        x = x + 1
+    }
+
+    x = x + 1
+
+    lambda()
+    lambda()
+
+    println(x.toString())
+
+    return lambda
+}
+
+class Integer(val value: Int) {
+    override fun toString() = value.toString()
+    operator fun plus(other: Int) = Integer(value + other)
+}