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.java

@@ -0,0 +1,11 @@
+// !forceNotNullTypes: false
+// !specifyLocalVariableTypeByDefault: true
+import java.util.HashSet;
+
+class Foo {
+  void foo(HashSet o) {
+    HashSet o2 = o;
+    int foo = 0;
+    foo = o2.size();
+  }
+}