`#[test]` attribute doesn't work correctly if used in front of `#[datatest::..]`

[idubrov]

Currently, the way #[test] is handled is proc macro deletes it when processing datatest attribute.

There are two reason #[test] attribute is desirable:

1. Better integration with IDEs which don't know about #[datatest], but can handle #[test] (for example, allow running test by clicking on it). I use IntelliJ as a test bed 😛
2. Differentiation between test tests and bench tests.

The issue is that if #[test] goes first, it will be handled by Rust built-in test harness, which is undesirable. Need to figure out a way to disable that -- potentially, #4 will solve that, too.