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_fnamemodify.vim b/src/testdir/test_fnamemodify.vim
index 63f2736..fd5f1ef 100644
--- a/src/testdir/test_fnamemodify.vim
+++ b/src/testdir/test_fnamemodify.vim
@@ -13,7 +13,7 @@
   call assert_equal('a', fnamemodify('../testdir/a', ':.'))
   call assert_equal('~/testdir/test.out', fnamemodify('test.out', ':~'))
   call assert_equal('~/testdir/a', fnamemodify('../testdir/a', ':~'))
-  call assert_equal('a', fnamemodify('../testdir/a', ':t'))
+  call assert_equal('a', '../testdir/a'->fnamemodify(':t'))
   call assert_equal('', fnamemodify('.', ':p:t'))
   call assert_equal('test.out', fnamemodify('test.out', ':p:t'))
   call assert_equal('out', fnamemodify('test.out', ':p:e'))