Kotlin: ensure test triggers _some_ work

This is required because of the changes made to ping.submit before and
how it was used internally.

Previously in event_database, when triggering startup event pings, we
only checked if storing a ping did not fail.
We did not check if any ping was actually submitted.
So the code around it launched a task on the workmanager, which then
subsequently checked and did nothing.
The test ensured that, and triggering the work manager was just fine
because there _was_ a pending task.

With the changes we now return `true` if any ping was submitted.
Or `false` if no event ping was submitted on startup.
Now on startup no task was triggered and thus no workmanager can be
triggered.
That made the test fail because `triggerWorkManager` requires a task to
be enqueued.

We can force a task by also recording into the builtin "events" ping.
It's a small hack, but allows us to test that late-registered pings with
events are correctly skipped AND their events are deleted.

glean-core/android/src/test/java/mozilla/telemetry/glean/private/EventMetricTypeTest.kt

@@ -495,7 +495,7 @@ class EventMetricTypeTest {
             category = "telemetry",
             name = "test_event",
             lifetime = Lifetime.Ping,
-            sendInPings = listOf(pingName),
+            sendInPings = listOf(pingName, "events"), // also send in builtin ping
             allowedExtraKeys = listOf("someExtra")
         )
 
@@ -523,11 +523,14 @@ class EventMetricTypeTest {
             sendIfEmpty = false,
             reasonCodes = listOf())
 
-        // Trigger worker task to upload the pings in the background
+        // Trigger worker task to upload the pings in the background.
+        // Because this also triggers the builtin "events" ping
+        // we should definitely get _something_.
         triggerWorkManager(context)
 
         // We can't properly test the absence of data,
         // but we can try to receive one and that causes an exception if there is none.
+        // We also get the "events" ping, which we'll simply ignore here.
         assertNull(waitForPingContent(pingName, null, server))
 
         // Now try to manually submit the ping.