Currently we use a marker that contains an `UnsafeCell` but `UnsafeCell`
is not `Sync` so this makes `io::Error` not `Sync`. We can instead wrap
the `UnsafeCell` and implement `Sync` for it.
Fix: #3883
The `api` test for types implementing `Send` and `Sync` is part of both
C-SEND-SYNC and also C-GOOD-ERR (for error types).
Refactor the Send/Sync tests in both `units` and `io` and improve
comments.