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.