Tests/admin/wpUserSearch: rename the original test class

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/65208

Rename the test class Tests_Admin_wpUserSearch to Admin_WpUserSearch_Construct_Test so that the name includes both the class under test (WP_User_Search) and the method being tested (__construct).

Replaces the @coversDefaultClass + relative @covers ::__construct() pattern with a single explicit class-level @covers WP_User_Search::__construct annotation. This is cleaner and compatible with PHPUnit 11.

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