test cargo fmt --all -- --check returns success after formatting

[gnzlbg]

Adds test for the failure in #2787 , doesn't fix anything.

[nrc]

Looks good thanks, but as expected, we now fail testing for stdsimd, so could you move that to the expect to fail list please?

[gnzlbg]

Yes, sorry, I was just checking why on failure travis was still reporting success (it seems like $0 in the ifs was reporting the exit code of tee which is always success... my bad...).

[gnzlbg]

I've tried to fix chalk by making it build, but it seems that it lacks permissions to generate the files on CI for whatever reason. I think that to make chalk pass fixing #2789 is probably a better way forward.

The build here should be green otherwise.

[nrc]

Thanks!