add failing test for 'dk' motion

test/normal-delete.sml

@@ -701,6 +701,30 @@ struct
              val expectedString = "\n"
              val actualString = LineGap.toString buffer
 
+             val expectedIdx = 0
+           in
+             Expect.isTrue
+               (expectedString = actualString andalso expectedIdx = cursorIdx)
+           end)
+    , test
+        "deletes newline and preceding line when cursor is second line \
+        \and second line contains only a newline"
+        (fn _ =>
+           let
+             (* arrange *)
+             val originalString = "hello\n\nagain\n"
+             val originalIdx = 6
+
+             val app = TestUtils.init originalString
+             val app = AppWith.idx (app, originalIdx)
+
+             (* act *)
+             val {buffer, cursorIdx, ...} = TestUtils.updateMany (app, "dk")
+
+             (* assert *)
+             val expectedString = "again\n"
+             val actualString = LineGap.toString buffer
+
              val expectedIdx = 0
            in
              Expect.isTrue