Build/Test Tools: Upgrades `Tests_Multisite_MS_Permalink_Collision` fixture methods and strict assertion.

Improvements include:

- Upgrades the test fixture methods to the new snake_case methods.
- Reorders the fixture methods for consistency.
- Uses strict assertions of assertSame and assertNotSame.

Follow-up to [51855-51856].

Props hellofromTonya.
See #51147.
Built from https://develop.svn.wordpress.org/trunk@51857


git-svn-id: http://core.svn.wordpress.org/trunk@51456 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51856';
+$wp_version = '5.9-alpha-51857';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.