patch 8.1.1993: 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_clientserver.vim b/src/testdir/test_clientserver.vim
index 2e4a66e..f358117 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -73,7 +73,7 @@
   call remote_send(name, ":call server2client(expand('<client>'), 'got it')\<CR>", 'g:myserverid')
   call assert_equal('got it', g:myserverid->remote_read(2))
 
-  call remote_send(name, ":call server2client(expand('<client>'), 'another')\<CR>", 'g:myserverid')
+  call remote_send(name, ":eval expand('<client>')->server2client('another')\<CR>", 'g:myserverid')
   let peek_result = 'nothing'
   let r = g:myserverid->remote_peek('peek_result')
   " unpredictable whether the result is already available.