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/createNestedClass.kt

@@ -0,0 +1,13 @@
+class A {
+    class B1
+    class B2(val x: Int)
+    class B3(val x: Long, val y: Int)
+    class B4(val str: String)
+}
+
+
+fun box(): String {
+    A.B1()
+    val b2 = A.B2(A.B3(42, 42).y)
+    return A.B4("OK").str
+}