Tests: Minimize the chances of signature conflicts for `assertEqualsWithDelta()`.

The PHPUnit 7.5+ method `assertEqualsWithDelta()` was polyfilled for PHPUnit < 7.5, but also overloaded for PHPUnit 7.5 itself, which was not necessary and created a higher chance of signature conflicts, especially when the WP test suite is used as a basis for integration tests with plugins/themes.

This change removes the unnecessary overloading for PHPUnit 7.5+ and simplifies the overloaded method for PHPUnit < 7.5, including removing the `IsEqual()` class alias declaration, no longer needed.

Follow-up to [48952].

Props jrf.
See #52625.
Built from https://develop.svn.wordpress.org/trunk@50986


git-svn-id: http://core.svn.wordpress.org/trunk@50595 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-alpha-50985';
+$wp_version = '5.8-alpha-50986';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.