patch 8.1.1921: 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_expr.vim b/src/testdir/test_expr.vim
index ccca792..9f43732 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -403,7 +403,7 @@
   call assert_equal(string(value), printf('%s', value))
 
   " funcref
-  call assert_equal('printf', printf('%s', function('printf')))
+  call assert_equal('printf', printf('%s', 'printf'->function()))
 
   " partial
   call assert_equal(string(function('printf', ['%s'])), printf('%s', function('printf', ['%s'])))
@@ -490,7 +490,7 @@
   endfunc
   call assert_equal(2, OneByName())
   call assert_equal(1, OneByRef())
-  let OneByRef = funcref('One')
+  let OneByRef = 'One'->funcref()
   call assert_equal(2, OneByRef())
   call assert_fails('echo funcref("{")', 'E475:')
 endfunc