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.
Because of the way grapheme breaking changes across updates to ICU and the Unicode standard, it may not even be legit to check this at all. It's certainly not unsafe to skip the check, so let's see if we can do that in release builds, as grapheme breaking is expensive.
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
