Auto merge of rust-lang#2207 - RalfJung:ui_test_parallelism, r=oli-obk

ui_test: ensure all worker threads stay around

Also organize files such that the by far slowest test (weak_memory/consistency) always starts first. It still finishes last on my system... even after I halved the iteration count.

Fixes rust-lang/miri#2204

Author: bors

tests/pass/weak_memory/consistency.rs renamed to tests/pass/0weak_memory_consistency.rs

@@ -214,7 +214,7 @@ fn test_single_thread() {
 }
 
 pub fn main() {
-    for _ in 0..100 {
+    for _ in 0..50 {
         test_single_thread();
         test_mixed_access();
         test_load_buffering_acq_rel();