fido-key-manager: disable tests

[micolous]

fido-key-manager requires elevation on Windows:

• For simplicity, this is declared in the application's manifest, included at link time. Elevation is handled by the platform before it calls .main.
• There are ways to conditionally elevate actions, but this is much more complicated to implement. The only thing this tool which doesn't require elevation is --help, so this is much simpler.

cargo can't launch binaries requiring elevation. There's also no way to detect at a build.rs level whether a binary target will be used for a test, and conditionally disable elevation support.

There's no tests in fido-key-manager anyway – so this just disables them explicitly so that a project-wide cargo test won't fail on Windows.