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.

git-svn-id: https://develop.svn.wordpress.org/trunk@52235 602fd350-edb4-49c9-b593-d223f7449a82

phpcs.xml.dist

@@ -249,6 +249,7 @@
 				<element value="WP_REST_Test_Controller"/>
 				<element value="WP_Image_Editor_Mock"/>
 				<element value="WP_Filesystem_MockFS"/>
+				<element value="Mock_Invokable"/>
 				<element value="MockPHPMailer"/>
 				<element value="MockAction"/>
 				<element value="WP_Object_Cache"/>