grpc-js resolver test using loopback6.unittest.grpc.io is flaky

[murgatroid99]

https://source.cloud.google.com/results/invocations/132ebe45-7193-4a46-9427-a97deb6c4c7d/targets

The test with loopback46.unittest.grpc.io also fails there, but it's not clear if those failures are independent.