minor #28673 [CS] Use combined assignment operators when possible (carusogabriel)

This PR was merged into the 2.8 branch.

Discussion
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[CS] Use combined assignment operators when possible

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | -   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | - <!-- required for new features -->

Less opcodes for us 😄

Commits
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c561e99394 [CS] Use combined assignment operators when possible

Author: nicolas-grekas

Tests/Util/StringUtilTest.php

@@ -35,7 +35,7 @@ public function testTrimUtf8Separators($hex)
 
         // Convert UCS-2BE to UTF-8
         $symbol = mb_convert_encoding($binary, 'UTF-8', 'UCS-2BE');
-        $symbol = $symbol."ab\ncd".$symbol;
+        $symbol .= "ab\ncd".$symbol;
 
         $this->assertSame("ab\ncd", StringUtil::trim($symbol));
     }