Rewrite SimpleUnbufferedChannelTest in channel_test.cc

[wangkuiyi]

The follow unit test has the title SimpleUnbufferedChannelTest

Paddle/paddle/framework/channel_test.cc

Lines 85 to 104 in 4716825

	TEST(Channel, SimpleUnbufferedChannelTest) {
	auto ch = MakeChannel<int>(0);
	unsigned sum_send = 0;
	std::thread t([&]() {
	for (int i = 0; i < 5; i++) {
	EXPECT_EQ(ch->Send(&i), true);
	sum_send += i;
	}
	});
	for (int i = 0; i < 5; i++) {
	int recv;
	EXPECT_EQ(ch->Receive(&recv), true);
	EXPECT_EQ(recv, i);
	}
	CloseChannel(ch);
	t.join();
	EXPECT_EQ(sum_send, 10U);
	delete ch;
	}

This implies that it is specifically design to test unbuffered channels.

However, the logic would pass with buffered channels too.

It seems that we should

1. either rename it to SimpleChannelTest -- remove the word "Unbuffered", or
2. rewrite it so that it works with unbuffered channel and fails with buffered channels.