Handle #[test]

[Lysxia]

Good rust projects have test suites, and it would be nice to be able to check assertions in test suites using creusot.

cargo creusot -- --tests on a random project fails with:

thread 'rustc' panicked at creusot/src/backend/ty_inv.rs:69:18:
not implemented: dyn [Binder { value: Trait(std::ops::Fn<(&mut test::Bencher,)>), bound_vars: [Region(BrNamed(DefId(26:1712 ~ test[7e83]::types::TestFn::'_#3), '_))] }, Binder { value: Projection(Output = std::result::Result<(), std::string::String>), bound_vars: [Region(BrNamed(DefId(26:1712 ~ test[7e83]::types::TestFn::'_#3), '_))] }, Binder { value: AutoTrait(DefId(2:32809 ~ core[98a5]::marker::Send)), bound_vars: [] }] + 'static

Those dyn seem to come from test::TestFn.

Maybe we can just tell creusot to ignore the generated test wrappers when --test is on.