Sync stable-2.5 test infrastructure with devel: - shippable.yml - test/runner/ - test/sanity/ - test/utils/ - docs/docsite/rst/dev_guide/testing/sanity/ - test/integration/targets/*/aliases
* Allow backoff for describe_subnets Improve exception handling to latest standards * Add integration test suite for ec2_vpc_subnet * Add test for creating subnet without AZ Fix bug identified by test Fixes #31905