Move out tests of a deprecated module to work around #[test] bugs

rust-lang#47238

Author: oli-obk

src/libstd/sync/mpsc/mod.rs

@@ -279,6 +279,9 @@ use self::select::StartResult;
 use self::select::StartResult::*;
 use self::blocking::SignalToken;
 
+#[cfg(all(test, not(target_os = "emscripten")))]
+mod select_tests;
+
 mod blocking;
 mod oneshot;
 mod select;