Use a non-zero exit code if the tests failed

The bots currently detect if the tests failed or not by checking the
output text, but this is error-prone as it would break if the text gets
changed (which is rather unexpected) and it's non-standard as usually
processes report success/failure via their exit code.

This commit makes sure that the test process fails with a non-zero exit
code if the tests failed, and a zero exit code otherwise. Note that we
keep the same output text as before, so this change should not impact
the bots, but it does form another step to unlock usage in GitHub
Actions (which uses the exit code of commands to determine the status of
the workflow run).

test/test.mjs

@@ -743,6 +743,7 @@ function onAllSessionsClosedAfterTests(name) {
     }
     var runtime = (Date.now() - startTime) / 1000;
     console.log(name + " tests runtime was " + runtime.toFixed(1) + " seconds");
+    process.exit(numErrors > 0 ? 1 : 0);
   };
 }