net/http: TestServerKeepAlivesEnabled_h2 failure with "test server has active conns"

[bcmills]

2021-02-19T00:03:44-a1e9148/linux-386-longtest

--- FAIL: TestServerKeepAlivesEnabled_h2 (2.01s)
    serve_test.go:5699: test server has active conns
FAIL
FAIL	net/http	63.805s

See previously #21724, #18701, #18127, #18083.

CC @bradfitz @empijei @neild

[bcmills]

Provisionally closing as fixed by CL 360854.

greplogs --dashboard -md -l -e 'test server has active conns' --since=2021-11-03