patch 8.2.0866: not enough tests for buffer writing

Problem:    Not enough tests for buffer writing.
Solution:   Add more tests. Use CheckRunVimInTerminal in more places.
            (Yegappan Lakshmanan, closes #6167)
diff --git a/src/testdir/test_signs.vim b/src/testdir/test_signs.vim
index ce122e8..fabf581 100644
--- a/src/testdir/test_signs.vim
+++ b/src/testdir/test_signs.vim
@@ -1734,9 +1734,7 @@
 
 " Test for correct cursor position after the sign column appears or disappears.
 func Test_sign_cursor_position()
-  if !CanRunVimInTerminal()
-    throw 'Skipped: cannot make screendumps'
-  endif
+  CheckRunVimInTerminal
 
   let lines =<< trim END
 	call setline(1, [repeat('x', 75), 'mmmm', 'yyyy'])