test: wait for uv.pipe_connect() callback (#37640)

Problem:
On Windows, writing to a pipe doesn't work if the pipe isn't connected
yet. This causes an RPC request to a session newly created by connect()
to hang, as it's waiting for a response to a request that never reaches
the server.

Solution:
Wait for uv.pipe_connect() callback to be called when using connect().

test/client/uv_stream.lua

@@ -67,8 +67,15 @@ function SocketStream.open(file)
     _stream_error = nil,
   }, SocketStream)
   uv.pipe_connect(socket, file, function(err)
+    uv.stop()
     self._stream_error = self._stream_error or err
   end)
+  -- On Windows, writing to the pipe doesn't work if it's not connected yet,
+  -- so wait for the connect callback to be called.
+  uv.run()
+  if self._stream_error then
+    error(self._stream_error)
+  end
   return self
 end
 

test/functional/ui/embed_spec.lua

@@ -284,7 +284,6 @@ describe('--embed UI', function()
   end)
 
   it('closing stdio with another remote UI does not leak memory #36392', function()
-    t.skip(t.is_os('win'), 'n.connect() hangs on Windows')
     clear({ args_rm = { '--headless' } })
     Screen.new()
     eq(1, #api.nvim_list_uis())
@@ -299,7 +298,6 @@ end)
 
 describe('--embed --listen UI', function()
   it('waits for connection on listening address', function()
-    t.skip(t.is_os('win'))
     clear()
     local child_server = assert(n.new_pipename())
     fn.jobstart({

test/functional/ui/messages_spec.lua

@@ -3350,7 +3350,6 @@ aliquip ex ea commodo consequat.]]
 end)
 
 it('pager works in headless mode with UI attached', function()
-  skip(is_os('win'), 'n.connect() hangs on Windows')
   clear()
   local child_server = assert(n.new_pipename())
   fn.jobstart({ nvim_prog, '--clean', '--headless', '--listen', child_server })