backend: add tests:

* test for uninitialized vals
* test initializers3: global `var` of reference type
* expression_as_statement (disabled): using type operator as statement

Also enable test break1.

backend.native/tests/runtime/basic/initializers3.kt

@@ -0,0 +1,7 @@
+class Foo(val bar: Int)
+
+var x = Foo(42)
+
+fun main(args: Array<String>) {
+    println(x.bar)
+}