add test against complete with leading bracket
add test against complete with leading bracket

add test: completes_multiple_use_bracket

This test has following mod file `modfile1`:

```
pub struct StarWars {
    pub Vadar: u8,
}; pub struct StarTrek {
    pub Spock: u8,
};";
```

and try to complete in these two case

this test will try to complete these two case:
`use modfile1::S`
`use modfile1::{S`

and test failed if these two case give different results

pmarcelll/racer,racer-rust/racer,birkenfeld/racer,phildawes/racer,jaxx/racer,alkama/racer,hotpxl/racer,sliz1/racer