8259954: gc/shenandoah/mxbeans tests fail with -Xcomp

Reviewed-by: rkennke, zgu

test/hotspot/jtreg/gc/shenandoah/mxbeans/TestChurnNotifications.java

@@ -159,7 +159,12 @@ public class TestChurnNotifications {
 
         System.gc();
 
+        // Wait until notifications start arriving, and then wait some more
+        // to catch the ones arriving late.
+        while (churnBytes.get() == 0) {
             Thread.sleep(1000);
+        }
+        Thread.sleep(5000);
 
         long actual = churnBytes.get();
 

test/hotspot/jtreg/gc/shenandoah/mxbeans/TestPauseNotifications.java

@@ -148,7 +148,12 @@ public class TestPauseNotifications {
             sink = new int[size];
         }
 
+        // Wait until notifications start arriving, and then wait some more
+        // to catch the ones arriving late.
+        while (pausesDuration.get() == 0) {
             Thread.sleep(1000);
+        }
+        Thread.sleep(5000);
 
         long pausesActual = pausesDuration.get();
         long cyclesActual = cyclesDuration.get();