x/net/http2: verify that net/http.Server.SetKeepAlivesEnabled(false) shuts down HTTP/2 server

[bradfitz]

Per #20239 (comment) and its immediate reply, it seems that https://go-review.googlesource.com/c/net/+/43230 might've broken net/http.Server.SetKeepAlivesEnabled(false) shutting down HTTP/2 server connections.

I guess there's no test for it.

/cc @tombergan @pam4

[bradfitz]

There is TestServerSetKeepAlivesEnabledClosesConns in serve_test.go, but it's only for HTTP/1.

[gopherbot]

Change https://golang.org/cl/122820 mentions this issue: net/http: remove dead code noted in post-submit review of CL 81778

[fraenkel]

This is actually difficult to test given how the current code works.
The h1ServerKeepAlivesDisabled checks for an interface which only works if its executed within the http package.