mcp: fix goroutine leaks in unit tests (#496)

This PR fixes goroutine leaks in all unit tests.

To find the leaks I integrated https://github.com/uber-go/goleak and I
suggest to keep using it to catch any future regressions.

I used the folowing script to more easily find individual tests which
were leaking goroutines:

```
for t in $(go test -run=Nothing -list=. ./mcp | grep -v ok); do go test ./mcp -run="$t" > /dev/null && echo -n . || echo -e "\nDetected leak: $t"; done
```

mcp/mcp_test.go

@@ -118,16 +118,6 @@ func TestEndToEnd(t *testing.T) {
 		t.Errorf("after connection, Clients() has length %d, want 1", len(got))
 	}
 
-	// Wait for the server to exit after the client closes its connection.
-	var clientWG sync.WaitGroup
-	clientWG.Add(1)
-	go func() {
-		if err := ss.Wait(); err != nil {
-			t.Errorf("server failed: %v", err)
-		}
-		clientWG.Done()
-	}()
-
 	loggingMessages := make(chan *LoggingMessageParams, 100) // big enough for all logging
 	opts := &ClientOptions{
 		CreateMessageHandler: func(context.Context, *CreateMessageRequest) (*CreateMessageResult, error) {
@@ -518,7 +508,9 @@ func TestEndToEnd(t *testing.T) {
 
 	// Disconnect.
 	cs.Close()
-	clientWG.Wait()
+	if err := ss.Wait(); err != nil {
+		t.Errorf("server failed: %v", err)
+	}
 
 	// After disconnecting, neither client nor server should have any
 	// connections.
@@ -626,6 +618,7 @@ func basicClientServerConnection(t *testing.T, client *Client, server *Server, c
 	if err != nil {
 		t.Fatal(err)
 	}
+	t.Cleanup(func() { _ = ss.Close() })
 
 	if client == nil {
 		client = NewClient(testImpl, nil)
@@ -634,6 +627,8 @@ func basicClientServerConnection(t *testing.T, client *Client, server *Server, c
 	if err != nil {
 		t.Fatal(err)
 	}
+	t.Cleanup(func() { _ = cs.Close() })
+
 	return cs, ss, func() {
 		cs.Close()
 		ss.Wait()
@@ -750,11 +745,7 @@ func TestMiddleware(t *testing.T) {
 		t.Fatal(err)
 	}
 	// Wait for the server to exit after the client closes its connection.
-	defer func() {
-		if err := ss.Wait(); err != nil {
-			t.Errorf("server failed: %v", err)
-		}
-	}()
+	t.Cleanup(func() { _ = ss.Close() })
 
 	var sbuf, cbuf bytes.Buffer
 	sbuf.WriteByte('\n')
@@ -773,7 +764,7 @@ func TestMiddleware(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer cs.Close()
+	t.Cleanup(func() { _ = cs.Close() })
 
 	if _, err := cs.ListTools(ctx, nil); err != nil {
 		t.Fatal(err)