chore: remove parallel running in integration tests (#8904)

# Which Problems Are Solved

Integration tests are flaky due to eventual consistency.

# How the Problems Are Solved

Remove t.Parallel so that less concurrent requests on multiple instance
happen. This allows the projections to catch up more easily.

# Additional Changes

- none

# Additional Context

- none

internal/notification/handlers/integration_test/telemetry_pusher_test.go

@@ -26,8 +26,6 @@ import (
 )
 
 func TestServer_TelemetryPushMilestones(t *testing.T) {
-	t.Parallel()
-
 	sub := sink.Subscribe(CTX, sink.ChannelMilestone)
 	defer sub.Close()