patch 8.1.1804: no test for display updating without a scroll region

Problem:    No test for display updating without a scroll region.
Solution:   Add a test.
diff --git a/src/testdir/check.vim b/src/testdir/check.vim
index 48dbd85..5d61f0d 100644
--- a/src/testdir/check.vim
+++ b/src/testdir/check.vim
@@ -37,3 +37,20 @@
     throw 'Skipped: only works on Unix'
   endif
 endfunc
+
+" Command to check that making screendumps is supported.
+" Caller must source screendump.vim
+command CheckScreendump call CheckScreendump()
+func CheckScreendump()
+  if !CanRunVimInTerminal()
+    throw 'Skipped: cannot make screendumps'
+  endif
+endfunc
+
+" Command to check that we can Run Vim in a terminal window
+command CheckRunVimInTerminal call CheckRunVimInTerminal()
+func CheckRunVimInTerminal()
+  if !CanRunVimInTerminal()
+    throw 'Skipped: cannot run Vim in a terminal window'
+  endif
+endfunc