patch 9.1.0415: Some functions are not tested

Problem:  Some functions are not tested
Solution: Add a few more tests, fix a few minor problems
          (Yegappan Lakshmanan)

closes: #14789

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 5adbb42..fd82801 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -4106,6 +4106,8 @@
     call assert_equal('', 'ὰ̳β̳́γ̳̂δ̳̃ε̳̄ζ̳̅'->slice(1, -6))
   END
   call v9.CheckLegacyAndVim9Success(lines)
+
+  call assert_equal(0, slice(v:true, 1))
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab