wp_constant

wp_constant.php

<?php

$wp_constant_mgr = new class WP_Constants_Manager();

function wp_constant( $name, $value ) {
    global $wp_constant_mgr;
    if ( UNIT_TEST_MODE )
        $wp_constant_mgr->set( $name, $value );
    else
        define( $name, $value );
}

function wp_defined( $name ) {
    global $wp_constant_mgr;
    if ( UNIT_TEST_MODE )
        return $wp_constant_mgr->get( $name );
    else
        return defined( $name );
}

final class WP_Constants_Manager {
    private $_constants = array();
    public function get( $name ) {
        return $this->_constants[ $name ] ?: false;
    }
    public function unset( $name ) {
        unset $this->_constants[ $name ];
    }
    public function set( $name, $value ) {
        if ( !isset( $this->_constants[ $name ] ) )
            $this->_constants[ $name ] = $value;
        else
            return WP_Error(0, "Constant '$name' already defined");
    }
}

Need to figure out "UNIT_TEST_MODE" and appropriate WP_Error code (or alternate error handling)

This doesn't account for pure constant usage (e.g. as a variable) either.