patch 8.0.1509: test for failing drag-n-drop command no longer fails

Problem:    Test for failing drag-n-drop command no longer fails.
Solution:   Check for the "dnd" feature.
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 05aceb3..27c5897 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -2176,7 +2176,11 @@
 endfunc
 
 func! Test_normal45_drop()
-  " basic test for :drop command
+  if !has('dnd')
+    return
+  endif
+
+  " basic test for drag-n-drop
   " unfortunately, without a gui, we can't really test much here,
   " so simply test that ~p fails (which uses the drop register)
   new