[TASK] Add coverage metadata to AtRuleTest (#1409)

As interfaces cannot be covered using an `@covers` annotation,
we need to use `@coversNothing` instead.

This will allow us to require coverage metadata for testcases
later down the road.

Author: oliverklee

tests/Unit/Property/AtRuleTest.php

@@ -7,6 +7,9 @@
 use PHPUnit\Framework\TestCase;
 use Sabberworm\CSS\Property\AtRule;
 
+/**
+ * @coversNothing
+ */
 final class AtRuleTest extends TestCase
 {
     /**