tests: Uncomment now valid GAT code behind FIXME

The code fails to parse with `nightly-2021-02-05`:

    $ cargo +nightly-2021-02-05 build
    error: generic associated types in trait paths are currently not implemented
     --> src/main.rs:9:42
      |
    9 | fn _bar<T: for<'a> StreamingIterator<Item<'a> = &'a [i32]>>(_iter: T) { /* ... */
      |                                          ^^^^

but parses with `nightly-2021-02-06`:

    $ cargo +nightly-2021-02-06 build
    warning: the feature `generic_associated_types` is incomplete and may not be safe to use and/or cause compiler crashes
    warning: 1 warning emitted

because it was (with high probability) fixed by PR 79554 which was merged
within that nightly range.

0 files changed, 0 insertions, 0 deletions