j2k: flatten test cases and testData directory structure

Move j2k/test/tests -> j2k/tests, j2k/test/testData -> j2k/testData

j2k/testData/fileOrElement/assignmentExpression/nullability.kt

@@ -0,0 +1,9 @@
+import java.util.HashSet
+
+class Foo {
+    fun foo(o: HashSet<Any>) {
+        val o2 = o
+        var foo = 0
+        foo = o2.size()
+    }
+}