Squiz/OperatorBracket: fix regression for new parent

Related to 3546 which fixed an inconsistency after 3484.

The change of the tokenization from `T_STRING` to `T_PARENT` for the `parent` keyword caused a regression in the `Squiz.Formatting.OperatorBracket` sniff.

Fixed now.

Includes unit tests.

Author: jrfnl

src/Standards/Squiz/Sniffs/Formatting/OperatorBracketSniff.php

@@ -142,6 +142,7 @@ public function process(File $phpcsFile, $stackPtr)
             T_THIS,
             T_SELF,
             T_STATIC,
+            T_PARENT,
             T_OBJECT_OPERATOR,
             T_NULLSAFE_OBJECT_OPERATOR,
             T_DOUBLE_COLON,

src/Standards/Squiz/Tests/Formatting/OperatorBracketUnitTest.inc

@@ -199,3 +199,5 @@ match ($a) {
     'b', 'c', 'd' => -2,
     default => -3,
 };
+
+$cntPages = ceil(count($items) / parent::ON_PAGE);

src/Standards/Squiz/Tests/Formatting/OperatorBracketUnitTest.inc.fixed

@@ -199,3 +199,5 @@ match ($a) {
     'b', 'c', 'd' => -2,
     default => -3,
 };
+
+$cntPages = ceil(count($items) / parent::ON_PAGE);