Laravel单元测试提示A facade root has not been set
在集成单元测试 tests/Unit 时遇到如下报错：
```
RuntimeException: A facade root has not been set.
/vendor/laravel/framework/src/Illuminate/Support/Facades/Facade.php:258
```
```
254▕ {
255▕ $instance = static::getFacadeRoot();
256▕
257▕ if (! $instance) {
258▕ throw new RuntimeException('A facade root has not been set.');
259▕ }
260▕
261▕ return $instance->$method(...$args);
262▕ }
```
修改 use PHPUnit\\Framework\\TestCase; 为 use Tests\\TestCase;
或者 use CreatesApplication 这个 trait
原因见原帖：[Laravel 6/7 testing: A facade root has not been set](https://stackoverflow.com/questions/61969956/laravel-6-7-testing-a-facade-root-has-not-been-set)
解决在单元测试中无法使用：Redis，DB，Config，Route 等 Facade 的问题。
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