patch 8.2.1241: cannot use getbufinfo() as a method

Problem:    Cannot use getbufinfo() as a method.
Solution:   Support using getbufinfo() as a method. (closes #6458)
diff --git a/src/testdir/test_bufwintabinfo.vim b/src/testdir/test_bufwintabinfo.vim
index 43d0473..eafadff 100644
--- a/src/testdir/test_bufwintabinfo.vim
+++ b/src/testdir/test_bufwintabinfo.vim
@@ -23,6 +23,9 @@
   call assert_equal('vim', l[0].variables.editor)
   call assert_notequal(-1, index(l[0].windows, '%'->bufwinid()))
 
+  let l = '%'->getbufinfo()
+  call assert_equal(bufnr('%'), l[0].bufnr)
+
   " Test for getbufinfo() with 'bufmodified'
   call assert_equal(0, len(getbufinfo({'bufmodified' : 1})))
   call setbufline('Xtestfile1', 1, ["Line1"])