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/api/grpc/system/integration_test/instance_test.go

@@ -15,8 +15,6 @@ import (
 )
 
 func TestServer_ListInstances(t *testing.T) {
-	t.Parallel()
-
 	isoInstance := integration.NewInstance(CTX)
 
 	tests := []struct {