Use the `PHPUnit\Framework\TestCase` notation instead of `PHPUnit_Framework_TestCase` while extending our TestCases. This will help us migrate to PHPUnit 6, that [no longer support snake case class names](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-6.0.md#changed-1).
`$cell->hasValidValue()` returns true if the cell has a value which conform to the rules defined in `$cell->getDataValidation()`. Closes #257