Migrate boxAgainstJava tests to multi-file framework

compiler/testData/codegen/boxAgainstJava/specialBuiltins/CharBuffer.kt

@@ -1,5 +0,0 @@
-fun box(): String {
-    val cb: CharBuffer = CharBuffer.impl()
-
-    return cb.get(0).toString() + (cb as CharSequence).get(1).toString()
-}

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

@@ -1,3 +1,5 @@
+// FILE: CharBuffer.java
+
 public abstract class CharBuffer implements CharSequence {
     public final int length() {
         return 0;
@@ -25,3 +27,11 @@ public abstract class CharBuffer implements CharSequence {
         };
     }
 }
+
+// FILE: 1.kt
+
+fun box(): String {
+    val cb: CharBuffer = CharBuffer.impl()
+
+    return cb.get(0).toString() + (cb as CharSequence).get(1).toString()
+}