Merge K2 box tests to main box tests

compiler/testData/codegen/box/properties/backingField/charSequenceWithBackingField5.kt

@@ -0,0 +1,16 @@
+// TARGET_BACKEND: JVM_IR
+// IGNORE_BACKEND_K1: JVM_IR
+
+open class Base {
+    open val x: CharSequence = "BASE"
+         // field = "BASE"
+}
+
+class Ok : Base() {
+    override val x: CharSequence
+        internal field: String = "OK"
+}
+
+fun box(): String {
+    return Ok().x
+}