Remove generated codegen tests, move all testData to box/

A single test file will be generated out of box/ directory

compiler/testData/codegen/box/innerNested/importNestedClass.kt

@@ -0,0 +1,18 @@
+import A.B
+import A.B.C
+
+class A {
+    class B {
+        class C
+    }
+}
+
+fun box(): String {
+    val a = A()
+    val b = B()
+    val ab = A.B()
+    val c = C()
+    val bc = B.C()
+    val abc = A.B.C()
+    return "OK"
+}