j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/methodCallExpression/collectionsMethods2.java

@@ -0,0 +1,8 @@
+import java.util.*;
+
+class A {
+    void foo() {
+        List<String> list = Collections.singletonList(null);
+        Set<String> set = Collections.singleton(null);
+    }
+}