The ResponseObjectTest and ProxiedSubresourceTest currently run:

executor = new InMemoryClientExecutor();

which internally does

ResteasyProviderFactory.getInstance()

So they basically end up running with the currently cached ResteasyProviderFactory in the JVM.
I've been able to see those tests fail because no MBW were available in the ResteasyProviderFactory, while running the tests one-by-one gives no error, hence the tests can work with a vanilla ResteasyProviderFactory instance.