Perf: Parallelize VDS instant update integration tests. (#1309)

* Perf: Parallelize VDS instant update integration tests.

* refactor: extract helper funcs for VDS instant update tests.

* Add await event watcher started before triggering rotation.

* Perf: Optimize Scale testing Infra for VDS. (#1317)

* perf: Share infra across parallel instant-update subtests.

* perf: Optimize Scale testing Infra for VDS Dynamic.

* Snapshot vds baseline before awaiting watcher start.

Author: Balaji2198