Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/collections/irrelevantImplCharSequence.kt

@@ -0,0 +1,28 @@
+// FILE: J.java
+
+public class J {
+    abstract static public class AImpl {
+        public char charAt(int index) {
+            return 'A';
+        }
+
+        public final int length() { return 56; }
+    }
+
+    public static class A extends AImpl implements CharSequence {
+        public CharSequence subSequence(int start, int end) {
+            return null;
+        }
+    }
+}
+
+// FILE: test.kt
+
+class X : J.A()
+
+fun box(): String {
+    val x = X()
+    if (x.length != 56) return "fail 1"
+    if (x[0] != 'A') return "fail 2"
+    return "OK"
+}