Do not depend on expects() for test stub implementation #6498

The implementation of the method method() of test test double API currently looks like this:

public function method(Constraint|PropertyHook|string $constraint): InvocationStubber
{
    return $this
        ->__phpunit_getInvocationHandler()
        ->expects(new AnyInvokedCount)
        ->method($constraint);
}

This means that the expects() method is called internally even for a test stub.

This is a remnant from the past when test stubs and mock objects were not as strictly separated as they are today.

We should explore whether we can refactor the runtime internals of the test stub functionality so that it does not require this, in an effort to further separate test stubs from mock objects.