Help us test the breaking bug-fix to Cargo features

Testing Rand since we have quite a few feature configurations and multiple crates.

I see this is no longer possible (though the command may not have worked correctly before anyway — I guess we have to test each crate individually):

$ cargo test --all --features=alloc -Z package-features
error: cannot specify features for more than one package

This happens to fix a “bug” we had when testing with the following command, plus “std” enabled by default in rand_core:

cargo test --tests --no-default-features --features=alloc --all

Other than that, no problems. I guess we just have to update our Travis scripts.