Auto merge of rust-lang#2848 - RalfJung:tokio, r=RalfJung

disable preemption in tokio tests again

The comment even still says we need preemption disabled, but the flag got lost in rust-lang/miri#2764.

Author: bors

src/tools/miri/tests/pass-dep/tokio/tokio_mvp.rs

@@ -1,5 +1,5 @@
 // Need to disable preemption to stay on the supported MVP codepath in mio.
-//@compile-flags: -Zmiri-permissive-provenance
+//@compile-flags: -Zmiri-permissive-provenance -Zmiri-preemption-rate=0
 //@only-target-x86_64-unknown-linux: support for tokio exists only on linux and x86
 
 #[tokio::main]