SplitHTTP client: Add xmux (multiplex controller) for H3 & H2 (#3613)

https://github.com/XTLS/Xray-core/pull/3613#issuecomment-2351954957

Closes https://github.com/XTLS/Xray-core/issues/3560#issuecomment-2247495778

---------

Co-authored-by: mmmray <142015632+mmmray@users.noreply.github.com>

transport/internet/splithttp/mux_test.go

@@ -0,0 +1,88 @@
+package splithttp_test
+
+import (
+	"context"
+	"testing"
+
+	. "github.com/xtls/xray-core/transport/internet/splithttp"
+)
+
+type fakeRoundTripper struct{}
+
+func TestMaxConnections(t *testing.T) {
+	config := Multiplexing{
+		MaxConnections: &RandRangeConfig{From: 4, To: 4},
+	}
+
+	mux := NewMuxManager(config, func() interface{} {
+		return &fakeRoundTripper{}
+	})
+
+	clients := make(map[interface{}]struct{})
+	for i := 0; i < 8; i++ {
+		clients[mux.GetResource(context.Background())] = struct{}{}
+	}
+
+	if len(clients) != 4 {
+		t.Error("did not get 4 distinct clients, got ", len(clients))
+	}
+}
+
+func TestCMaxReuseTimes(t *testing.T) {
+	config := Multiplexing{
+		CMaxReuseTimes: &RandRangeConfig{From: 2, To: 2},
+	}
+
+	mux := NewMuxManager(config, func() interface{} {
+		return &fakeRoundTripper{}
+	})
+
+	clients := make(map[interface{}]struct{})
+	for i := 0; i < 64; i++ {
+		clients[mux.GetResource(context.Background())] = struct{}{}
+	}
+
+	if len(clients) != 32 {
+		t.Error("did not get 32 distinct clients, got ", len(clients))
+	}
+}
+
+func TestMaxConcurrency(t *testing.T) {
+	config := Multiplexing{
+		MaxConcurrency: &RandRangeConfig{From: 2, To: 2},
+	}
+
+	mux := NewMuxManager(config, func() interface{} {
+		return &fakeRoundTripper{}
+	})
+
+	clients := make(map[interface{}]struct{})
+	for i := 0; i < 64; i++ {
+		client := mux.GetResource(context.Background())
+		client.OpenRequests.Add(1)
+		clients[client] = struct{}{}
+	}
+
+	if len(clients) != 32 {
+		t.Error("did not get 32 distinct clients, got ", len(clients))
+	}
+}
+
+func TestDefault(t *testing.T) {
+	config := Multiplexing{}
+
+	mux := NewMuxManager(config, func() interface{} {
+		return &fakeRoundTripper{}
+	})
+
+	clients := make(map[interface{}]struct{})
+	for i := 0; i < 64; i++ {
+		client := mux.GetResource(context.Background())
+		client.OpenRequests.Add(1)
+		clients[client] = struct{}{}
+	}
+
+	if len(clients) != 1 {
+		t.Error("did not get 1 distinct clients, got ", len(clients))
+	}
+}