j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/issues/kt-792-mixed.java

@@ -0,0 +1,12 @@
+//file
+package demo;
+
+class Test {
+  Test(Integer i) {
+  }
+
+  void test() {
+    int i = 10;
+    new Test(i);
+  }
+}