Symfony version(s) affected: 4.1

Description
#26702 introduces an annoying BC break:
when using Prophecy, it's not possible to prohesize our interfaces anymore.

How to reproduce

    $serializerException = $this->prophesize(ExceptionInterface::class);
    $serializerException->willExtend(\Exception::class);

    $serializerException->reveal();

Was working before, now throws Prophecy\Exception\Doubler\MethodNotExtendableException: Method getMessage is not extendable, so can not be added.

Possible Solution
Reverting #26702. (I'm trying to figure another solution).