patch 8.1.1909: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make a few more functions usable as a method.
diff --git a/src/testdir/test_bufwintabinfo.vim b/src/testdir/test_bufwintabinfo.vim
index 0e8c7d1..7f0a6ce 100644
--- a/src/testdir/test_bufwintabinfo.vim
+++ b/src/testdir/test_bufwintabinfo.vim
@@ -18,7 +18,7 @@
     let l = getbufinfo('%')
     call assert_equal(bufnr('%'), l[0].bufnr)
     call assert_equal('vim', l[0].variables.editor)
-    call assert_notequal(-1, index(l[0].windows, bufwinid('%')))
+    call assert_notequal(-1, index(l[0].windows, '%'->bufwinid()))
 
     " Test for getbufinfo() with 'bufmodified'
     call assert_equal(0, len(getbufinfo({'bufmodified' : 1})))