move arc_drop test to miri-test-libstd

and make weak memory consistency test a bit faster again

Author: RalfJung

tests/pass/0concurrency_arc_drop.rs

@@ -217,7 +217,7 @@ fn test_single_thread() {
 }
 
 pub fn main() {
-    for _ in 0..75 {
+    for _ in 0..50 {
         test_single_thread();
         test_mixed_access();
         test_load_buffering_acq_rel();