chore: ignore failing unit tests in network management

[JoeWang1127]

In this PR:

• Ignore failing unit tests in network management.

This is a temporary fix of googleapis/sdk-platform-java#1839.

Confirm the change can generate @Ignore annotation to failing tests:

pip install library_generation
python library_generation/generate_repo.py generate --generation-config-yaml="$(pwd)"/generation_config.yaml --target-library-api-shortname=networkmanagement

For simplicity, I copied library_generation directory in sdk-platform-java to my local google-cloud-java repo.