minor #16187 [2.8][tests] Use @requires annotation when possible (nicolas-grekas)

This PR was merged into the 2.8 branch.

Discussion
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[2.8][tests] Use @requires annotation when possible

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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814d961 [tests] Use @requires annotation when possible

Author: nicolas-grekas

Tests/Util/StringUtilTest.php

@@ -24,13 +24,10 @@ public function testTrim()
 
     /**
      * @dataProvider spaceProvider
+     * @requires extension mbstring
      */
     public function testTrimUtf8Separators($hex)
     {
-        if (!function_exists('mb_convert_encoding')) {
-            $this->markTestSkipped('The "mb_convert_encoding" function is not available');
-        }
-
         // Convert hexadecimal representation into binary
         // H: hex string, high nibble first (UCS-2BE)
         // *: repeat until end of string