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_expand.vim b/src/testdir/test_expand.vim
index bd8021f..53f753d 100644
--- a/src/testdir/test_expand.vim
+++ b/src/testdir/test_expand.vim
@@ -58,7 +58,7 @@
   call assert_equal('e Xfile1', expandcmd('e %'))
   edit Xfile2
   edit Xfile1
-  call assert_equal('e Xfile2', expandcmd('e #'))
+  call assert_equal('e Xfile2', 'e #'->expandcmd())
   edit Xfile2
   edit Xfile3
   edit Xfile4