vim-patch:8.2.0261: some code not covered by tests

Problem:    Some code not covered by tests.
Solution:   Add test cases. (Yegappan Lakshmanan, closes vim/vim#5645)
f0cee1971f

Cherry-pick Test_bunload_with_offset() from patch 8.2.0243

src/nvim/testdir/test_mapping.vim

@@ -31,6 +31,7 @@ func Test_abclear()
 
    abclear
    call assert_equal("\n\nNo abbreviation found", execute('abbrev'))
+   call assert_fails('%abclear', 'E481:')
 endfunc
 
 func Test_abclear_buffer()