Bug 1619269 - Enable alerts for jsshell-bench tests r=perftest-reviewers,davehunt

Differential Revision: https://phabricator.services.mozilla.com/D85748

testing/jsshell/benchmark.py

@@ -28,7 +28,7 @@ with `ac_add_options --enable-js-shell` or specify it with `--binary`.
 class Benchmark(object):
     __metaclass__ = ABCMeta
     lower_is_better = True
-    should_alert = False
+    should_alert = True
 
     def __init__(self, shell, args=None, shell_name=None):
         self.shell = shell