Re-enable skipped test but mark it slow

The test was taking more than a minute before CL
https://crrev.com/c/2259933. Afterwards, it takes a lot longer, but I
could never reproduce a hang locally.
Let's re-enable and mark SLOW to increase the chance that it runs to
completion. We can then see how much slower it really got.

Also add some output that helps triaging in case it really hangs.

R=ahaas@chromium.org

Bug: v8:9506
Change-Id: I09a935ca0018517d45c6c008a099b8052bc45c47
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2273117
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68582}

test/mjsunit/mjsunit.status

@@ -291,6 +291,7 @@
   'array-constructor': [PASS, SLOW],
   'json': [PASS, SLOW],
   'large-object-literal-slow-elements': [PASS, SLOW],
+  'harmony/sharedarraybuffer-worker-gc-stress': [PASS, SLOW],
 
   # BUG(v8:4779): Crashes flakily with stress mode on arm64.
   'array-splice': [PASS, SLOW, ['arch == arm64', NO_VARIANTS]],
@@ -307,9 +308,6 @@
 
   # BUG(v8:10035)
   'compiler/deopt-array-builtins': [SKIP],
-
-  # BUG(v8:9506): times out.
-  'harmony/sharedarraybuffer-worker-gc-stress': [SKIP],
 }],  # 'gc_stress'
 
 ##############################################################################

test/mjsunit/worker-ping-test.js

@@ -114,12 +114,15 @@ function RunWorkerPingTest(config) {
       if (msg.thing) {
         thing = msg.thing;
       } else if (msg.error) {
+        print('Error in worker:', msg.error);
         worker.terminate();
         throw msg.error;
       }
     }
   }
+  print('Terminating workers.');
   for (let worker of workers) {
     worker.terminate();
   }
+  print('Workers terminated.');
 }