limit tokio test to linux

Author: oli-obk

tests/fail/tokio_mvp.rs

@@ -1,6 +1,7 @@
 //@compile-flags: -Zmiri-disable-isolation
 //@error-pattern: can't call foreign function: epoll_create1
 //@normalize-stderr-test: "   = note: inside .*\n" -> ""
+//@only-target-linux: the errors differ too much between platforms
 
 #[tokio::main]
 async fn main() {}