j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/assignmentExpression/nullability-settings.kt

@@ -0,0 +1,11 @@
+// !forceNotNullTypes: false
+// !specifyLocalVariableTypeByDefault: true
+import java.util.HashSet
+
+class Foo {
+    fun foo(o: HashSet<Any?>?) {
+        val o2: HashSet<Any?>? = o
+        var foo: Int = 0
+        foo = o2!!.size()
+    }
+}