RFR: EvilSyncBug test is too slow

[Aleksey Shipilev]

This test never failed for years. Seems odd to stress use it with very long runs (takes ~10 minutes
in sh/jdk8u with new asserts). Let's trim its run time down:

diff -r 96802db391ff test/hotspot/jtreg/gc/shenandoah/EvilSyncBug.java
--- a/test/hotspot/jtreg/gc/shenandoah/EvilSyncBug.java Fri Sep 07 12:56:42 2018 -0400
+++ b/test/hotspot/jtreg/gc/shenandoah/EvilSyncBug.java Sat Sep 08 14:33:01 2018 +0200
@@ -41,5 +41,5 @@
 public class EvilSyncBug {

-    private static final int NUM_RUNS = 200;
+    private static final int NUM_RUNS = 20;

     static Thread[] hooks = new MyHook[10000];

Testing: EvilSyncBug with sh/jdk8u

-Aleksey