[Test] Properly merge box against java testdata into codegen/box

Previous commit about it was 3199ce03 and it was completely wrong

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

@@ -0,0 +1,29 @@
+// DONT_TARGET_EXACT_BACKEND: JS JS_IR JS_IR_ES6 WASM NATIVE
+// MODULE: lib
+// 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"; }
+}
+
+// MODULE: main(lib)
+// FILE: 1.kt
+
+fun box(): String {
+    return f(JavaClass())
+}
+
+internal fun f(c: C) = c.ok