patch 8.2.0261: some code not covered by tests

Problem:    Some code not covered by tests.
Solution:   Add test cases. (Yegappan Lakshmanan, closes #5645)
diff --git a/src/testdir/test_mapping.vim b/src/testdir/test_mapping.vim
index 00dfc5b..71f254a 100644
--- a/src/testdir/test_mapping.vim
+++ b/src/testdir/test_mapping.vim
@@ -38,6 +38,7 @@
 
    abclear
    call assert_equal("\n\nNo abbreviation found", execute('abbrev'))
+   call assert_fails('%abclear', 'E481:')
 endfunc
 
 func Test_abclear_buffer()