HTML API: Fix "${var} in strings" deprecation notice in html5lib test.

Changeset [58712] introduced the following compile time PHP deprecation notice on >= PHP 8.2 test runs:

{{{
Deprecated: Using ${var} in strings is deprecated, use {$var} instead in /var/www/tests/phpunit/tests/html-api/wpHtmlProcessorHtml5lib.php on line 257
PHPUnit 9.6.20 by Sebastian Bergmann and contributors.
}}}

The `${` syntax for string interpolation was deprecated in PHP 8.2 and should not be used anymore.

Ref: https://wiki.php.net/rfc/deprecate_dollar_brace_string_interpolation

Follow-up to [58712].

Props jrf.
See #61530, #59654, #61576.

git-svn-id: https://develop.svn.wordpress.org/trunk@58733 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/html-api/wpHtmlProcessorHtml5lib.php

@@ -254,7 +254,7 @@ private static function build_tree_representation( ?string $fragment_context, st
 		}
 
 		if ( '' !== $text_node ) {
-			$output .= "${text_node}\"\n";
+			$output .= "{$text_node}\"\n";
 		}
 
 		// Tests always end with a trailing newline.