func TestSubtests(t *testing.T) {
    routes := []struct {
        url  string
        path string
    }{
        {"http://example.com/1", "/1"},
        {"http://example.com/2", "/2"},
        {"http://example.com/3", "/3"},
        {"http://example.com/4", "/4"},
        {"http://example.com/5", "/5"},
    }

    t.Run("sequential", func(t *testing.T) {
        for _, tt := range routes {
            t.Run(tt.url, func(t *testing.T) {
                u, _ := url.Parse(tt.url)
                if u.Path != tt.path {
                    t.Errorf("expected %v, got %v", tt.path, u.Path)
                }
            })
        }
    })

    t.Run("parallel", func(t *testing.T) {
        for _, tt := range routes {
            t.Run(tt.url, func(t *testing.T) {

                t.Parallel() // <== trying to set Parallel(), while using tt from the range loop

                u, _ := url.Parse(tt.url)
                if u.Path != tt.path {
                    t.Errorf("expected %v, got %v", tt.path, u.Path)
                }
            })
        }
    })
}

    for _, tt := range routes {
        go func() {
            fmt.Println(tt.url) // Not guaranteed which item will be stored in tt during the goroutine execution.
        }()
    }

    for _, tt := range routes {
        go func(url string) {
            fmt.Println(url)
        }(tt.url)
    }