Write more testcases for the fnc_tree parser

Those get handled incorrectly rn:
```rust
#[no_mangle]
fn myFn(a: [f64; 1]) { unimplemented!() }

#[no_mangle]
fn myBn(a: (f64, i32)) { unimplemented!() }
```

These got fixed in the meantime. Probably worth adding to rustc as regression tests.
```rust

#[no_mangle]
fn myDn(a: [f64; 2]) { unimplemented!() }

#[no_mangle]
fn mygn(a: (f64, f64, f64)) { unimplemented!() }
```