[PHP 8.3] test: update expectation for highlight code

[kenjis]

Description
See #7850

E.g.,

1) CodeIgniter\Helpers\TextHelperTest::testHighlightCode
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'<code><span style="color: #000000">
-<span style="color: #0000BB">&lt;?php&nbsp;var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$this</span><span style="color: #007700">);&nbsp;</span><span style="color: #0000BB">?&gt;&nbsp;</span>
-</span>
-</code>'
+'<pre><code style="color: #000000"><span style="color: #0000BB">&lt;?php var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$this</span><span style="color: #007700">); </span><span style="color: #0000BB">?&gt; ?&gt;</span></code></pre>'
/home/runner/work/CodeIgniter4/CodeIgniter4/tests/system/Helpers/TextHelperTest.php:267

https://github.com/codeigniter4/CodeIgniter4/actions/runs/6035726283/job/16376653028?pr=7886

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide