patch 8.1.1925: 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_getvar.vim b/src/testdir/test_getvar.vim
index 3b61d68..e9868c7 100644
--- a/src/testdir/test_getvar.vim
+++ b/src/testdir/test_getvar.vim
@@ -31,7 +31,7 @@
   let t:other = 777
   let def_list = [4, 5, 6, 7]
   tabrewind
-  call assert_equal([1, 2, 3], gettabvar(3, 'var_list'))
+  call assert_equal([1, 2, 3], 3->gettabvar('var_list'))
   call assert_equal([1, 2, 3], gettabvar(3, 'var_list', def_list))
   call assert_equal({'var_list': [1, 2, 3], 'other': 777}, gettabvar(3, ''))
   call assert_equal({'var_list': [1, 2, 3], 'other': 777},
@@ -61,7 +61,7 @@
   let def_dict = {'dict2': 'newval'}
   wincmd b
   tabrewind
-  call assert_equal({'dict': 'tabwin'}, gettabwinvar(2, 3, 'var_dict'))
+  call assert_equal({'dict': 'tabwin'}, 2->gettabwinvar(3, 'var_dict'))
   call assert_equal({'dict': 'tabwin'},
 				\ gettabwinvar(2, 3, 'var_dict', def_dict))
   call assert_equal({'var_dict': {'dict': 'tabwin'}}, gettabwinvar(2, 3, ''))