j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/NullableMethodDotAccess.java

@@ -0,0 +1,10 @@
+//file
+class C {
+    String getString(boolean b) {
+        return b ? "a" : null;
+    }
+
+    int foo() {
+        return getString(true).length();
+    }
+}