Test failures on pc-windows-gnu

As of the most recent nightly, *rustc 1.42.0-nightly (01a46509a 2019-12-20)*, our Windows builder in Travis has begun failing. https://travis-ci.com/dtolnay/linkme/jobs/269503401

Both `cargo test` and `cargo test --release` fail on Windows with the following:

```console
thread 'test' panicked at 'assertion failed: `(left == right)`
left: `0`,
right: `3`', tests\distributed_slice.rs:17:5
```

The relevant code in the test file is:

https://github.com/dtolnay/linkme/blob/5376b495bcc7587102b35ded301fc87e2e09fb26/tests/distributed_slice.rs#L3-L17

Mentioning @kabergstrom in case this is something you might be able to look into.