test: Extract a common "netns service" base class

[wkz]

Description

We have a bunch of things that create processes that run inside a netns (DHCP servers, tcpdump instances etc.)

These share a bunch of behavior:

• Creates and manages a subprocess (with proper termination semantics etc.)
• Should be usable as context managers
• Should be usable with explicit .start()/.stop() methods
• ...maybe more?

It would be good if we had a common implementation for this.

Additional Information

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General Information

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