[CLEANUP] Use getAllDeclarationBlocks in colorParsing test

... instead of `getAllRuleSets`.

This avoids testing if `RuleSet`s are `DeclarationBlock`s, and will be needed
for #1194.

Author: JakeQZ

tests/ParserTest.php

@@ -91,10 +91,7 @@ public function files(): void
     public function colorParsing(): void
     {
         $document = self::parsedStructureForFile('colortest');
-        foreach ($document->getAllRuleSets() as $ruleSet) {
-            if (!($ruleSet instanceof DeclarationBlock)) {
-                continue;
-            }
+        foreach ($document->getAllDeclarationBlocks() as $ruleSet) {
             $selectors = $ruleSet->getSelectors();
             $selector = $selectors[0]->getSelector();
             if ($selector === '#mine') {