patch 8.2.1593: tests do not check the error number properly

Problem:    Tests do not check the error number properly.0
Solution:   Add a colon after the error number. (closes #6869)
diff --git a/src/testdir/test_terminal2.vim b/src/testdir/test_terminal2.vim
index f9f221b..a99818b 100644
--- a/src/testdir/test_terminal2.vim
+++ b/src/testdir/test_terminal2.vim
@@ -36,9 +36,9 @@
   call StopVimInTerminal(buf)
   call delete('Xwinsize')
 
-  call assert_fails('set termwinsize=40', 'E474')
-  call assert_fails('set termwinsize=10+40', 'E474')
-  call assert_fails('set termwinsize=abc', 'E474')
+  call assert_fails('set termwinsize=40', 'E474:')
+  call assert_fails('set termwinsize=10+40', 'E474:')
+  call assert_fails('set termwinsize=abc', 'E474:')
 
   set termwinsize=
 endfunc