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/nestedGeneric.kt

@@ -0,0 +1,12 @@
+class Outer {
+    class Nested<T>(val t: T) {
+        fun box() = t
+    }
+}
+
+fun box(): String {
+    if (Outer.Nested<String>("OK").box() != "OK") return "Fail"
+    
+    val x: Outer.Nested<String> = Outer.Nested("OK")
+    return x.box()
+}