Allow replacement of test-case assertion on per-fn scale

Currently `test_case` allows replacing it's built in `assert` via `using` and `with` keywords.

We'd like to be able to replace assertion on per-function and global levels. This refers to requirements specified within https://github.com/frondeus/test-case/issues/31.

Initial work specifies
```
#[test_case(input2)]
#[test_case_use(assert_nan)]
fn tested(v: X) -> f64 {
...
}
```
as possible solution for per-function replacements.