test: fix removed helper

Author: dcampos

doc/snippy.txt

@@ -518,6 +518,7 @@ expand_options ~
         Default: {}
 
 virtual_markers ~
+    Requires nvim >= `0.10.0`.
     Enable virtual text markers that show all available (inactive) tabstops
     and placeholders.
         Type: `Table`

test/functional/core_spec.lua

@@ -4,7 +4,8 @@ local clear, command, eval = helpers.clear, helpers.command, helpers.eval
 local feed, alter_slashes = helpers.feed, helpers.alter_slashes
 local insert = helpers.insert
 local eq, neq, ok = helpers.eq, helpers.neq, helpers.ok
-local sleep, exec_lua = helpers.sleep, helpers.exec_lua
+local sleep = vim and vim.uv.sleep or helpers.sleep
+local exec_lua = helpers.exec_lua
 
 describe('Core', function()
     local screen