net/http/httptest: deadlock in Server.Close on plan9

[millerresearch]

#!watchflakes
post <- builder ~ `^plan9` && pkg == "net/http" && `test timed out` && `blocked in Close`

This is a peculiar one. So far just one occurrence at https://build.golang.org/log/042a896ae485c11f420d43237a20b30bf7aa54f9 running TestResponseControllerSetFutureWriteDeadline.

It appears the server is waiting for the listener goroutine, which is blocked in a syscall opening a tcp connection's listen file. No other significant goroutines are running. If the test has actually ended, the open should have been cancelled. Not clear why this didn't happen.