x/net/nettest: add TestListener API

[mdlayher]

I'm currently implementing my own net.Listener (see also #30984 and #31033), https://godoc.org/github.com/mdlayher/vsock#Listener, and would like to ensure it is in full compliance with the net.Listener contract; that is:

• Accept blocks until a connection is receive, or it is interrupted
• Close terminates the listener, but can also unblock Accept
• Maybe: test for SetDeadline method (which is common for net.Listener, see proposal: net/v2: add SetDeadline to Listener interface #6892) and verify that an expired deadline unblocks Accept as well (this idea is in line with what I propose with x/net/nettest: extend TestConn with optional interface checks #31033)

I've played around with this a bit locally to try to see what makes sense, and I will send a draft CL with my proposed API and a single test. The basic idea is to mirror what TestConn is doing, and perhaps the two can share a fair bit of internal code.

/cc @dsnet @mikioh @acln0

[gopherbot]

Change https://golang.org/cl/169177 mentions this issue: nettest: draft implementation of TestListener APIs

[mikioh]

Just FYI: https://go-review.googlesource.com/c/net/+/123056

[mdlayher]

I had no idea this was already in progress. I'll take a look tomorrow.

[mikioh]

I still need to process long-standing CLs and that's one of them, sorry for my laziness. Basically, making x/net/nettest useful sounds nice because we are entering the modular era; vendoring x/net/nettest could deduplicate the test harnesses in the package net of the standard library.

[mdlayher]

Happy to help with that effort.

[rsc]

This seems fine to add to x/net/nettest. Will leave to @mikoh and @mdlayher to work out the review of CL 123056. Thanks!