test(lsp): make async format test work properly (#35794)

Overriding vim.lsp.handlers['textDocument/formatting'] doesn't work here
because fake_lsp_server_setup() uses a table with __index to specify
client handlers, which takes priority over vim.lsp.handlers[], and as a
result the overridden handler is never called, and the test ends before
the vim.wait() even finishes.

Instead, set a global variable from the handler that is actually reached
(by vim.rpcrequest() from client handler), and avoid stopping the event
loop too early.

test/functional/plugin/lsp_spec.lua

@@ -5313,22 +5313,18 @@ describe('LSP', function()
                 notify_msg = msg
               end
 
-              local handler = vim.lsp.handlers['textDocument/formatting']
-              local handler_called = false
-              vim.lsp.handlers['textDocument/formatting'] = function()
-                handler_called = true
-              end
-
+              _G.handler_called = false
               vim.lsp.buf.format({ bufnr = bufnr, async = true })
               vim.wait(1000, function()
-                return handler_called
+                return _G.handler_called
               end)
 
               vim.notify = notify
-              vim.lsp.handlers['textDocument/formatting'] = handler
-              return { notify = notify_msg, handler_called = handler_called }
+              return { notify_msg = notify_msg, handler_called = _G.handler_called }
             end)
             eq({ handler_called = true }, result)
+          elseif ctx.method == 'textDocument/formatting' then
+            exec_lua('_G.handler_called = true')
           elseif ctx.method == 'shutdown' then
             client:stop()
           end