test(lsp): fix flakiness in inline completion test (neovim#35676)

The flakiness happens because get() uses vim.schedule(), and a following
key may be processed before the scheduled event. Use poke_eventloop() to
ensure that the scheduled event is processed.

Author: zeertzjq

test/functional/plugin/lsp/inline_completion_spec.lua

@@ -181,6 +181,7 @@ describe('vim.lsp.inline_completion', function()
       exec_lua(function()
         vim.lsp.inline_completion.get()
       end)
+      n.poke_eventloop()
       feed('<Esc>')
       screen:expect({ grid = grid_applied_candidates })
     end)
@@ -267,6 +268,7 @@ describe('vim.lsp.inline_completion', function()
       exec_lua(function()
         vim.lsp.inline_completion.get()
       end)
+      n.poke_eventloop()
       feed('<Esc>')
       screen:expect([[
         function fibonacci(n) {                              |