futures: tests/try_join.rs: Don't break w/o features

Only really appropriate action here is to gate the whole file with
`#![cfg()]`

Tests now build and pass with all valid feature combinations

See previous commits for details and rationale

Author: kentfredric

futures/tests/try_join.rs

@@ -1,3 +1,5 @@
+#![cfg(feature = "executor")] // executor::
+#![cfg(feature = "async-await")] // try_join!
 #![deny(unreachable_code)]
 
 use futures::{try_join, executor::block_on};