5893: Allow running a test as a binary r=matklad a=jonas-schievink
If a test uses `harness = false`, it just contains an `fn main` that is executed via `cargo test`. This adds support for that.
Note though that Cargo doesn't actually tell us whether `harness = false`, so this hint will always show up when you put an `fn main` into an integration test. Normally people shouldn't be doing that if they do use the harness though.
Co-authored-by: Jonas Schievink <jonas.schievink@ferrous-systems.com>
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