Build/Test Tools: Fix warnings from `stdClass::__invoke()` callback mocks in REST API tests.

When running core tests on PHPUnit 8.x and 9.x, four non-blocking warnings were displayed for the REST API tests:

{{{
There were 4 warnings:

1) Tests_REST_Request::test_route_level_validate_callback
createPartialMock called with method(s) __invoke that do not exist in stdClass. This will not be allowed in future versions of PHPUnit.

2) Tests_REST_Request::test_route_level_validate_callback_no_parameter_callbacks
createPartialMock called with method(s) __invoke that do not exist in stdClass. This will not be allowed in future versions of PHPUnit.

3) Tests_REST_Request::test_route_level_validate_callback_is_not_executed_if_parameter_validation_fails
createPartialMock called with method(s) __invoke that do not exist in stdClass. This will not be allowed in future versions of PHPUnit.

4) Tests_REST_Server::test_callbacks_are_not_executed_if_request_validation_fails
createPartialMock called with method(s) __invoke that do not exist in stdClass. This will not be allowed in future versions of PHPUnit.
}}}

These warnings are due to the PHP native `stdClass` not having a `__invoke()` method declared.

This commit adds a `Mock_Invokable` reusable class and replaces the `stdClass` with this new class.

Follow-up to [48945], [48947].

Props sourovroy, jrf.
Fixes #53844.
Built from https://develop.svn.wordpress.org/trunk@52235


git-svn-id: http://core.svn.wordpress.org/trunk@51827 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52234';
+$wp_version = '5.9-alpha-52235';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.