[RFC] Adding unit test for rlibc

We are pulling in rlibc into Zinc (hackndev/zinc#113), but while reading the code, I though it could do with some unit tests.

So I have added some tests here, but seem like there are a few issues... Please see comments with `XXX` and respond if you have any thoughts :+1:
The error message without the extra attribute was this:
```
lib.rs:117:13: 117:22 error: macro undefined: 'assert_eq!'
```

I am now actually face with something more interesting:
```
error: type `&'static str` does not implement any method in scope named `len`
```