Can’t programmatically log user in php

This modified version of your code works perfectly for me:

function test_login() {
    if ( empty( $_GET['uname'] ) || is_user_logged_in() ) {
        return;
    }

    $user = wp_signon( [
        'user_login'    => sanitize_text_field( wp_unslash( $_GET['uname'] ) ),
        'user_password' => 'password',
        'remember'      => true,
    ] );

    if ( is_wp_error( $user ) ) {
        wp_die( $user->get_error_message() );
    } else {
        wp_clear_auth_cookie(); // clear any user cookies set in the browser
        wp_set_current_user( $user->ID, $user->user_login ); // set current site user
        wp_set_auth_cookie( $user->ID ); // set user cookie
        do_action( 'wp_login', $user->ID, $user ); // trigger core action.
    }
}
add_action( 'after_setup_theme', 'test_login' );