j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/issues/kt-5294.java

@@ -0,0 +1,11 @@
+import java.util.List;
+
+class X {
+    private final List<Y> list;
+
+    X(List<Y> list) {
+        this.list = list;
+    }
+
+    class Y{}
+}