Parse '-' in numeric values used in test case

For:

#[test_case(15, 15)]
#[test_case(-15, 15)]
```rust
fn test_abs(value: f64, expected: f64) {
assert_eq!(value.abs(), expected)
}
```

We get error:

> error[E0428]: the name `_15_15` is defined multiple times
> --> src/test.rs
> |
> | #[test_case(15, 15)]
> | ^^^^^^^^^^^^^^^^^^^^
> | |
> | previous definition of the value `_15_15` here
> | `_15_15` redefined here
> |
> = note: `_15_15` must be defined only once in the value namespace of this mod

Start by reproducing the issue with the shown #[test_case] attributes and test_abs example, then trace the test_case procedural macro's handling of numeric arguments and generated names. Done means negative and positive numeric values compile without duplicate-name errors and the generated tests both run with the expected assertions.

Written by the indexing model from the issue text.