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

nj2k/testData/multiFile/GetterAndSetterUsages/external/3.kt.expected

@@ -0,0 +1,13 @@
+import test.AAA
+
+fun foo1(a: AAA) {
+    a.x = a.x + 1
+}
+
+fun foo2(a: AAA?) {
+    a?.x = (a?.x ?: 0) + 1
+}
+
+fun AAA.foo() {
+    x = x + 1
+}