Allow platform specific tests

Author: oli-obk

src/tools/tidy/src/pal.rs

@@ -58,6 +58,7 @@ const EXCEPTION_PATHS: &[&str] = &[
     "src/libstd/sys_common/net.rs",
     "src/libterm", // Not sure how to make this crate portable, but test crate needs it.
     "src/libtest", // Probably should defer to unstable `std::sys` APIs.
+    "src/libstd/sync/mpsc", // some tests are only run on non-emscripten
 
     // std testing crates, okay for now at least
     "src/libcore/tests",