New J2K: add multi-file conversion tests from old j2k

nj2k/testData/multiFile/GetterAndSetterUsages/external/4.java

@@ -0,0 +1,7 @@
+package test;
+
+class JavaClass {
+    void foo(AAA a) {
+        a.setX(a.getX() + 1);
+    }
+}