cache/integration-tests needs cache/tag-interop for some
reason. cache/tag-interop needs PHP>=5.5 for some reason.

I can still install and run these tests if I ignore these
platform requirements, but I don't want to do that (phpunit
also depends on packages that are properly versioned and
would install an incorrect version if I ignore platform
requirements...)

So I need to install cache/integration-tests separately.
But the test relies on this class existing. I'll just
create a stub if it doesn't exist.

I'm getting sick and tired of this...