patch 8.1.1996: 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_functions.vim b/src/testdir/test_functions.vim
index 1d8d74d..3588c76 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -1180,7 +1180,7 @@
   let save_shell = &shell
   set shell=bash
   call assert_equal("'text'", shellescape('text'))
-  call assert_equal("'te\"xt'", shellescape('te"xt'))
+  call assert_equal("'te\"xt'", 'te"xt'->shellescape())
   call assert_equal("'te'\\''xt'", shellescape("te'xt"))
 
   call assert_equal("'te%xt'", shellescape("te%xt"))