test: add missing vim.schedule mock to selection test

Fixes test failures caused by missing vim.schedule function in the
local vim mock used by selection_test.lua. The logger now requires
vim.schedule to be available for all logging levels.

Change-Id: Iec504a23187522a4f7968ec88d85476719a177d4
Signed-off-by: Thomas Kosiewski <[email protected]>

Author: ThomasK33

tests/selection_test.lua

@@ -3,6 +3,9 @@ if not _G.vim then
     schedule_wrap = function(fn)
       return fn
     end,
+    schedule = function(fn)
+      fn()
+    end,
     _buffers = {},
     _windows = {},
     _commands = {},