[korowai/testing] Implement constraint that compares arrays after sorting them by keys

Examples:

```php
assertEqualsKsorted(['a' => 'Z', 'b' => 'Y'], ['b' => 'Y', 'a' => 'Z']); // pass
assertEqualsKsorted(['a' => 'Z', 'b' => 'Y'], ['a' => 'Z']); // fail
```
Implementation concept:
```php
ksort($expect);
ksort($actual);
assertEquals($expect, $actual);
```