[Pre-RFC] `cargo package` should include fewer files by default

kpreid:

Most packages’ tests do not take up significant amounts of disk space (or have any non-code files at all). Keeping tests, when they are cheap, is valuable for “wait, did this published package ever work as intended? If so, on which Rust versions?” kinds of investigations. I also think that it would be valuable if cargo publish verification ran tests to reduce the chances of publishing a broken-as-published package — all the more so if the default file exclusion rules were stricter.

Certainly it would be bad if we can't use crater any more because most things end up with broken tests.