patch 8.1.1888: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
diff --git a/src/testdir/test_arglist.vim b/src/testdir/test_arglist.vim
index fc086fa..a382a1a 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -90,8 +90,8 @@
   call assert_equal('', bufname('%'))
   call assert_equal(1, line('$'))
   rew
-  call assert_notequal(curbuf, bufnr('%'))
-  call assert_equal('Xargadd', bufname('%'))
+  call assert_notequal(curbuf, '%'->bufnr())
+  call assert_equal('Xargadd', '%'->bufname())
   call assert_equal(2, line('$'))
 
   call delete('Xargadd')