[Test] Properly merge box against java testdata into codegen/box

Previous commit about it was 3199ce03 and it was completely wrong

compiler/testData/codegen/box/specialBuiltins/charBuffer.kt

@@ -0,0 +1,41 @@
+// DONT_TARGET_EXACT_BACKEND: JS JS_IR JS_IR_ES6 WASM NATIVE
+// IGNORE_BACKEND_FIR: JVM_IR
+// MODULE: lib
+// FILE: CharBuffer.java
+
+public abstract class CharBuffer implements CharSequence {
+    public final int length() {
+        return 0;
+    }
+
+    public final char charAt(int index) {
+        return 'K';
+    }
+
+    // The key problem here is that `get` has the same signature as kotlin.CharSequence.get but completely different semantics
+    public abstract char get(int index);
+    public abstract CharBuffer subSequence(int start, int end);
+
+    public static CharBuffer impl() {
+        return new CharBuffer() {
+            @Override
+            public char get(int index) {
+                return 'O';
+            }
+
+            @Override
+            public CharBuffer subSequence(int start, int end) {
+                return null;
+            }
+        };
+    }
+}
+
+// MODULE: main(lib)
+// FILE: 1.kt
+
+fun box(): String {
+    val cb: CharBuffer = CharBuffer.impl()
+
+    return cb.get(0).toString() + (cb as CharSequence).get(1).toString()
+}