Test builds should default to a different executable name

Packaging papercut.

    erustc "${MAIN}" 
    # Force the output name to avoid overwriting the one we just compiled
    expecting_tests && erustc --test -o "${WORK}/test-${CID}" "${MAIN}"

[brson]

Makes sense to me.

[steveklabnik]

Triage: no change that i'm aware of.

[Mark-Simulacrum]

Is this a backwards compatible change? Technically, it's likely that people rely on the current behavior, though it isn't quite ideal.

[Mark-Simulacrum]

Closing as a backwards incompatible change -- though I think it would be desirable to change things here, it seems like too small a gain given that we'd be breaking backwards compat.