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_balloon_gui.vim b/src/testdir/test_balloon_gui.vim
index 5b486c0..70a88f3 100644
--- a/src/testdir/test_balloon_gui.vim
+++ b/src/testdir/test_balloon_gui.vim
@@ -12,7 +12,7 @@
   call balloon_show('')
 
   let msg = 'that that'
-  call balloon_show(msg)
+  eval msg->balloon_show()
   call assert_equal(msg, balloon_gettext())
   sleep 10m
   call balloon_show('')