patch 8.1.1953: more functions can be used as a method

Problem:    More functions can be used as a method.
Solution:   Allow more functions to be used as a method.
diff --git a/src/testdir/test_syn_attr.vim b/src/testdir/test_syn_attr.vim
index 27f9fc0..cc20678 100644
--- a/src/testdir/test_syn_attr.vim
+++ b/src/testdir/test_syn_attr.vim
@@ -3,7 +3,7 @@
 func Test_missing_attr()
   hi Mine term=bold cterm=italic
   call assert_equal('Mine', synIDattr(hlID("Mine"), "name"))
-  call assert_equal('', synIDattr(hlID("Mine"), "bg", 'term'))
+  call assert_equal('', synIDattr("Mine"->hlID(), "bg", 'term'))
   call assert_equal('1', synIDattr(hlID("Mine"), "bold", 'term'))
   call assert_equal('1', synIDattr(hlID("Mine"), "italic", 'cterm'))
   hi Mine term=reverse cterm=inverse
@@ -12,7 +12,7 @@
   hi Mine term=underline cterm=standout gui=undercurl
   call assert_equal('1', synIDattr(hlID("Mine"), "underline", 'term'))
   call assert_equal('1', synIDattr(hlID("Mine"), "standout", 'cterm'))
-  call assert_equal('1', synIDattr(hlID("Mine"), "undercurl", 'gui'))
+  call assert_equal('1', synIDattr("Mine"->hlID(), "undercurl", 'gui'))
   hi Mine term=NONE cterm=NONE gui=NONE
   call assert_equal('', synIDattr(hlID("Mine"), "bold", 'term'))
   call assert_equal('', synIDattr(hlID("Mine"), "italic", 'cterm'))