Enable/disable P2P per test instance to avoid race conditions
Changes:
- Track enabled P2P pairs in member variable enabledP2PPairs
- SetUp: Only record pairs WE successfully enabled (both SUCCESS)
- TearDown: Disable P2P bidirectionally for our pairs, ignore errors
- Removes global P2P state dependency between test instances
Works for both:
- 2 physical GPUs duplicated 4× (8 logical devices)
- 8 distinct physical GPUs
Fixes #19033
