[chore][processor/groupbytrace] fix flaky TestEventShutdown (#44548)

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Ex. Adding a feature - Explain what this achieves.-->
#### Description

We should wait for the shutdown call to return before checking for
expired events, because the shutdown goroutine might not be able to
acquire the lock in a timely manner.

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#### Link to tracking issue
Fixes
#44533

Author: VihasMakwana

processor/groupbytraceprocessor/event_test.go

@@ -425,16 +425,16 @@ func TestEventShutdown(t *testing.T) {
 	// verify
 	assert.Equal(t, int64(1), traceReceivedFired.Load())
 
+	// wait until the shutdown has returned
+	shutdownWg.Wait()
+
 	// If the code is wrong, there's a chance that the test will still pass
 	// in case the event is processed after the assertion.
 	// Verify that the expired event is not processed (should remain 0)
 	assert.Eventually(t, func() bool {
 		return traceExpiredFired.Load() == 0
 	}, 100*time.Millisecond, 5*time.Millisecond)
 	assert.Equal(t, int64(0), traceExpiredFired.Load())
-
-	// wait until the shutdown has returned
-	shutdownWg.Wait()
 }
 
 func TestPeriodicMetrics(t *testing.T) {