Certain traps can't be supported on WASI, such as expectations for crashes, `setjmp` use, dynamic linking, multi-threading etc. These tests are marked as `UNSUPPORTED` when targeting Wasm/WASI.
This decreases total testing time by over a minute on my old Mac Pro.
It probably has much less effect on systems with fewer cores, but shouldn't
be any worse there.
Swift SVN r22745