Migrate boxWithJava tests to multi-file framework

compiler/testData/codegen/boxWithJava/number.kt

@@ -0,0 +1,33 @@
+// FILE: FortyTwoExtractor.java
+
+public class FortyTwoExtractor {
+    private Number fortyTwo = new FortyTwo();
+
+    public int intValue() {
+        return fortyTwo.intValue();
+    }
+}
+
+// FILE: FortyTwoExtractor.kt
+
+class FortyTwo : Number() {
+    override fun toByte() = 42.toByte()
+
+    override fun toShort() = 42.toShort()
+
+    override fun toInt() = 42
+
+    override fun toLong() = 42L
+
+    override fun toFloat() = 42.0f
+
+    override fun toDouble() = 42.0
+
+    override fun toChar()  = 42.toChar()    
+}
+
+fun box(): String {
+    val extractor = FortyTwoExtractor()
+    if (extractor.intValue() != 42) return "FAIL"
+    return "OK"
+}