Code Modernization: Explicitly declare all properties in WP_Test_Stream.

Dynamic (non-explicitly declared) properties are deprecated as of PHP 8.2 and are expected to become a fatal error in PHP 9.0.

The `WP_Test_Stream` class is a stream wrapper for use in the tests and must comply with the PHP requirements for such stream wrappers.

In this case, the class did not declare the required public `$context` property, which led to deprecation notices about the property being dynamically created from the `Tests_Image_Editor_Imagick::test_streams()` and `Tests_Image_Meta::test_stream()` tests.

Reference: [https://www.php.net/manual/en/class.streamwrapper.php#streamwrapper.props PHP Manual: streamWrapper: Properties].

Follow-up to [49230], [50771], [53557], [53558], [53850], [53851], [53852], [53853], [53854], [53856], [53916], [53935], [53936], [53937], [53938], [53942], [53945], [53948].

Props jrf, antonvlasenko.
See #56033.

git-svn-id: https://develop.svn.wordpress.org/trunk@53949 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/class-wp-test-stream.php

@@ -30,6 +30,15 @@ class WP_Test_Stream {
 	public $bucket;
 	public $data_ref;
 
+	/**
+	 * The current context.
+	 *
+	 * @link https://www.php.net/manual/en/class.streamwrapper.php
+	 *
+	 * @var resource|null
+	 */
+	public $context;
+
 	/**
 	 * Initializes internal state for reading the given URL.
 	 *