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/testutil.lua

@@ -199,31 +199,34 @@ function M.test_rpc_server(config)
   })
   --- @type integer, integer
   local code, signal
+  local busy = 0
+  local exited = false
   local function on_request(method, args)
-    if method == 'setup' then
-      if config.on_setup then
-        config.on_setup()
-      end
-      return NIL
+    busy = busy + 1
+    if method == 'setup' and config.on_setup then
+      config.on_setup()
     end
-    if method == 'init' then
-      if config.on_init then
-        config.on_init(client, unpack(args))
-      end
-      return NIL
+    if method == 'init' and config.on_init then
+      config.on_init(client, unpack(args))
     end
-    if method == 'handler' then
-      if config.on_handler then
-        config.on_handler(unpack(args))
-      end
+    if method == 'handler' and config.on_handler then
+      config.on_handler(unpack(args))
+    end
+    busy = busy - 1
+    if busy == 0 and exited then
+      stop()
     end
     return NIL
   end
   local function on_notify(method, args)
     if method == 'exit' then
       code, signal = unpack(args)
-      return stop()
+      exited = true
+      if busy == 0 then
+        stop()
+      end
     end
+    return NIL
   end
   --  TODO specify timeout?
   --  run(on_request, on_notify, nil, 1000)