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_cmdline.vim b/src/testdir/test_cmdline.vim
index f0d83c6..6ed7848 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -209,7 +209,7 @@
   endif
   let groupcount = len(getcompletion('', 'event'))
   call assert_true(groupcount > 0)
-  let matchcount = len(getcompletion('File', 'event'))
+  let matchcount = len('File'->getcompletion('event'))
   call assert_true(matchcount > 0)
   call assert_true(groupcount > matchcount)