patch 8.0.1653: screen dump is made too soon

Problem:    Screen dump is made too soon.
Solution:   Wait until the ruler is displayed. (Ozaki Kiichi, closes #2755)
diff --git a/src/testdir/screendump.vim b/src/testdir/screendump.vim
index 87a5823..d4fae07 100644
--- a/src/testdir/screendump.vim
+++ b/src/testdir/screendump.vim
@@ -59,6 +59,9 @@
   let buf = term_start(cmd, {'curwin': 1, 'term_rows': rows, 'term_cols': cols})
   call assert_equal([rows, cols], term_getsize(buf))
 
+  " Wait for the ruler (in the status line) to be shown.
+  call WaitFor({-> len(term_getline(buf, rows)) >= cols - 1})
+
   return buf
 endfunc