Recent CNI specification changes require the container ID on ADD/DEL,
which the testcases were not providing. Fix that up so things work
when this repo gets CNI revendored.
Namespace creation had an unergonomic interface and isn't used, except
for testing code. Remove it; downstream users should really be creating
their own namespaces
- start list of linux_only plugins; ignore them when testing on Windows
- Isolate linux-only code by filename suffix
- Remove stub (NotImplemented) functions
- other misc. fixes for Windows compatibility
