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/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({