Tests: Consistently create a post fixture in old date or slug redirec…

…t tests.

This affects:
* `Tests_Rewrite_OldDateRedirect`
* `Tests_Rewrite_OldSlugRedirect`

This commit updates the latter test class to create a post in the `wpSetUpBeforeClass()` method, for consistency with the former class. This ensures that both classes declare the `$post_id` property as `static`, to avoid a situation where non-static access is accidentally used when copying similar test cases from one class to the other.

Follow-up to [34659], [42587], [54077].

See #55652.
Built from https://develop.svn.wordpress.org/trunk@54078

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-54077';
+$wp_version = '6.1-alpha-54078';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.