Test generated manifests against rustup

[Diggsey]

Several times, PRs have changed the format of the manifests in such a way that they can no longer be installed by rustup. This results in the corresponding nightlies being uninstallable, and doesn't give much time to fix it before the next nightly: rust-lang/rustup#1092 (comment)

We should add tests to catch these problems before they occur.

cc @brson @alexcrichton

[ranma42]

cc me

[alexcrichton]

Seems like a good idea! The best location for this may actually be the final integration point, rust-central-station as that's where the manifest is actually generated. If someone would like to implement this I can try to help guide you through the steps!

[Mark-Simulacrum]

Closing in favor of rust-lang/rust-central-station#17