patch 8.1.1176: test for dragging a tab is flaky

Problem:    Test for dragging a tab is flaky.
Solution:   Add a brief sleep.
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index 42a38d2..ddde7da 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -325,6 +325,8 @@
         \              'Tab page 2',
         \              '    Xtab1'], a)
 
+    " brief sleep to avoid causing a double-click
+    sleep 20m
     %bwipe!
   endfor