input.c: Process only safe events before blocking.

Introduce multiqueue_process_priority() to process only events at or
above a certain priority.

test/functional/api/vim_spec.lua

@@ -9,7 +9,6 @@ local funcs = helpers.funcs
 local request = helpers.request
 local meth_pcall = helpers.meth_pcall
 local command = helpers.command
-local wait = helpers.wait
 
 describe('api', function()
   before_each(clear)
@@ -222,13 +221,6 @@ describe('api', function()
     end)
   end)
 
-  local function appendfile(fname, text)
-    local file = io.open(fname, 'a')
-    file:write(text)
-    file:flush()
-    file:close()
-  end
-
   describe('nvim_get_mode', function()
     it("during normal-mode `g` returns blocking=true", function()
       nvim("input", "o")                -- add a line