add test badge

Author: Insolita

README.md

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 #### Yii2 validator for complex array structures
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+![yii2-array-structure-validator](https://.com/Insolita/yii2-array-structure-validator/workflows/yii2-array-structure-validator/badge.svg?branch=master)
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 Validator for array attributes, unlike builtin "each" validator, that support only one rule, this validator can
 * validate multiple array attributes and even nested data structures
 * All keys that should be present in array must be described, for optional keys default value should be set