Add fixtures in Yii2 module (yiisoft#182)

To be able to follow the instruction on https://www.yiiframework.com/doc/guide/2.0/en/test-fixtures `$I->grabFixture()` we need to enable the fixtures feature of the Yii2. Methinks this would be a good thing to enable by default.

Author: Maarten van Middelaar

tests/unit.suite.yml

@@ -8,4 +8,4 @@ modules:
  enabled:
  - Asserts
  - Yii2:
- part: [orm, email]
+ part: [orm, email, fixtures]