patch 8.2.0243: insufficient code coverage for ex_docmd.c functions

Problem:    Insufficient code coverage for ex_docmd.c functions.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5618)
diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index e858d72..2ecc520 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -295,6 +295,8 @@
   close!
   call delete('Xtest')
   bwipe! Xtest
+
+  call assert_fails('earlier xyz', 'E475:')
 endfunc
 
 func Test_insert_expr()