workaround

main_test.go

package main

import (
        "sync"
        "testing"
)

func TestAppend(t *testing.T) {
        x := make([]string, 0, 6)
        x = append(x, "start")

        wg := sync.WaitGroup{}
        wg.Add(2)
        go func() {
                defer wg.Done()
                y := make([]string, 0, len(x)+2)
                y = append(y, x...)
                y = append(y, "hello", "world")
                t.Log(cap(y), len(y), y[0])
        }()
        go func() {
                defer wg.Done()
                z := make([]string, 0, len(x)+2)
                z = append(z, x...)
                z = append(z, "goodbye", "bob")
                t.Log(cap(z), len(z), z[0])
        }()
        wg.Wait()
}