Migrate boxAgainstJava tests to multi-file framework

compiler/testData/codegen/boxAgainstJava/syntheticExtensions/getter.kt

@@ -1,3 +1,11 @@
+// FILE: JavaClass.java
+
+class JavaClass {
+    public String getOk() { return "OK"; }
+}
+
+// FILE: 1.kt
+
 fun box(): String {
     return JavaClass().ok
 }