Migrate boxAgainstJava tests to multi-file framework

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

@@ -1,3 +1,24 @@
+// FILE: C.java
+
+interface A {
+    String getOk();
+}
+
+interface B {
+    String getOk();
+}
+
+interface C extends A, B {
+}
+
+// FILE: JavaClass.java
+
+class JavaClass implements C {
+    public String getOk() { return "OK"; }
+}
+
+// FILE: 1.kt
+
 fun box(): String {
     return f(JavaClass())
 }