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_cmdline.vim b/src/testdir/test_cmdline.vim
index 6ebfa31..628db67 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -843,7 +843,7 @@
   f %<
   call assert_equal('foo-B', expand('%'))
   new
-  call assert_fails('f #<', 'E95')
+  call assert_fails('f #<', 'E95:')
   bw!
   f foo-B.txt
   f %<-A