Make integration tests more tolerant of transient failures

Our integration tests often perform "non-idempotent" operations, but these can fail even if our code is working fine. A typical test may have this structure:

```cc
auto create = client.CreateFoo(....);
ASSERT_STATUS_OK(create);

auto update = client.UpdateFoo(...);
ASSERT_STATUS_OK(update);

auto del = client.DeleteFoo(...);
ASSERT_STATUS_OK(del);
```

None of those operations are retried by default, as the operations are non-idempotent. I think we could rewirte the tests as follows:

```cc
auto create = client.CreateFoo(... Options{}.set(RetryEverything()));
ASSERT_THAT(create, StatusIs(AnyOf(StatusCode::kOk, StatusCode::kAlreadyExists)));
```

Or if we don't want to retry the operation, we can write it as:

```cc
auto create = client.CreateFoo(...);
if (IsTransient(...)) GTEST_SKIP() << create.status();
```

This probably requires a mini-design doc, and will require breaking down the work per test.

For now I am going to consolidate all existing "transient error on non-idempotent operation" flakes to this one.

该 issue 未指定文件或具体测试；请先查看此处汇总的现有 transient-error-on-non-idempotent-operation flakes。起草所要求的 mini-design，然后按每个集成测试拆分工作，决定每个操作在发生暂时性失败时应重试还是跳过；除使这些测试更加宽容之外，未指定其他完成标准。

由索引模型根据 Issue 内容生成。