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_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 88141a9..a4b4f53 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -121,7 +121,7 @@
 
   " Test for failure when the alternate buffer/file no longer exists.
   edit Xfoo | %bw
-  call assert_fails(':wincmd ^', 'E23')
+  call assert_fails(':wincmd ^', 'E23:')
 
   " Test for the expected behavior when we have two named buffers.
   edit Xfoo | edit Xbar
@@ -154,9 +154,9 @@
 
   %bwipeout
   let l:nr = bufnr('%') + 1
-  call assert_fails(':execute "normal! ' . l:nr . '\<C-W>\<C-^>"', 'E92')
-  call assert_fails(':' . l:nr . 'wincmd ^', 'E16')
-  call assert_fails(':0wincmd ^', 'E16')
+  call assert_fails(':execute "normal! ' . l:nr . '\<C-W>\<C-^>"', 'E92:')
+  call assert_fails(':' . l:nr . 'wincmd ^', 'E16:')
+  call assert_fails(':0wincmd ^', 'E16:')
 
   edit Xfoo | edit Xbar | edit Xbaz
   let l:foo_nr = bufnr('Xfoo')