Hide #[test] functions by default in emacs

[brson]

It would be nice if emacs did 'folding' to hide the bodies of test functions. Tests are very convenient to write inline, and it's great to put tests as close to the functions they test as possible, but having tests littered everywhere makes reading the code later not as fun.

[mpenet]

I am not sure it is a good idea to have this part of the mode itself, it is a very uncommon/specific feature for a language mode imho, but you can probably achieve this using folding-mode or something similar (I am told it is really easy to setup).

[brson]

Agreed it doesn't belong in rust-mode, and it sounds like folding-mode can do this trivially, so there's nothing to do here. Thanks, @mpenet.