cargo test -p ignores dev-dependencies

[brson]

In rustup I have a rustup crate that depends on a rustup-dist crate.

In the Cargo.toml for rustup-dist I have:

[dev-dependencies]
url = "1.1.0"

And in my 'tests' folder tests with import the url crate.

When I run tests from the 'workspace' level with cargo test -p rustup-dist the url crate does not resolve, but if I cd into the rustup-dist directory and run cargo test it works correctly.

[alexcrichton]

Dupe of #860, but definitely an unfortunate bug. Will be fixed with workspaces!