`rust_test()` doesn't set `OUT_DIR`

[rejuvenile]

My use case is calling a Rust macro in my test which expects to find generated code relative to the OUT_DIR compilation-time environment variable. It's likely that rust_binary() doesn't set this either; I haven't checked because I use a genrule to create the output files.

[rejuvenile]

Note, I don't use cargo_build_script() here due to compilation difficulties.