--logfile for libtest doesn't respect --format.

[daxpedda]

Observed behaviour
When using for example cargo bench -- -Z unstable-options --format json --logfile bench.json, bench.json has the usual human output format. Stdout prints JSON correctly though.

Expected behaviour
I expect to see JSON output in the file, exactly like in stdout.

Notes
I know at the moment rust is working on the custom test framework (#50297), but other people are still relying on the current features (#51924, rust-lang/cargo#5670).
In any case, I think this is a bug, right? Would love to make a PR.

Rustc
rustc 1.33.0-nightly (a7be40c 2018-12-26)