fix: association concurrently appending

[black-06]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Fix bug when association concurrently appending.

Association in each goroutine will modify the user.Languages by reflect.Append, which method grows the slice.
If it grows successfully, languages will be a new slice, that's ok.
But If cap is enough, these goroutines will write the same slice, that's wrong.

If we only call DB.FirstOrCreate(&user), user.Languages len is 0 and cap is 0, that's ok.
if we only call DB.Preload("Languages").FirstOrCreate(&user), still len 0 and cap 0.
But we call DB.FirstOrCreate(&user) first, then DB.Preload("Languages").FirstOrCreate(&user), user.Languages is len 0 and cap 10, this error will occur #5801

User Case Description

DB.FirstOrCreate(&user)
DB.Preload("Languages").FirstOrCreate(&user)

var wg sync.WaitGroup
for i := 0; i < count; i++ {
wg.Add(1)
go func(user User, language Language) {
	err := DB.Model(&user).Association("Languages").Append(&language)
	AssertEqual(t, err, nil)

	wg.Done()
}(user, languages[i])
}
wg.Wait()

[black-06]

When I added a new test, the old test failed. Did I miss anything?

[a631807682]

When we use goroutine, another connection is created, causing PRAGMA foreign_keys to become invalid.
https://www.sqlite.org/foreignkeys.html#fk_enable

We can create a db instance specifically for it

	tx, err := OpenTestConnection()
	// your test