Merge K2 box tests to main box tests

compiler/testData/codegen/box/properties/backingField/charSequenceWithBackingField3.kt

@@ -0,0 +1,12 @@
+// TARGET_BACKEND: JVM_IR
+// IGNORE_BACKEND_K1: JVM_IR
+
+class Base {
+    val x: CharSequence
+        internal field: String = "OK"
+
+}
+val s: String get() = Base().x
+fun box(): String {
+    return s
+}