Suggestion: `loadFixtures()`: Change the default for `append` to false

The default mode when loading fixtures is to **purge**: https://symfony.com/bundles/DoctrineFixturesBundle/current/index.html#loading-fixtures

> By default the `load` command purges the database

But with `$I->loadFixtures()`, the default is to **append**. I think this is confusing and should be changed. However, I don't see any easy upgrade path... ;-)

首先，在模块中找到 `$I->loadFixtures()` 的实现和测试。将其当前的 append 默认值与文档中所述的 purge 行为进行比较，然后确定更改默认值会如何影响现有用法，以及哪些测试应定义新行为。

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