net/http: TestServerKeepAlivesEnabled_h{1,2} failures with "got same connection between first and second requests"

[bcmills]

2021-06-10T12:41:37-dc00dc6/illumos-amd64

--- FAIL: TestServerKeepAlivesEnabled_h1 (0.01s)
    serve_test.go:5703: got same connection between first and second requests
FAIL
FAIL	net/http	15.430s

2021-05-02T18:22:19-0d32d9e/illumos-amd64
2020-11-13T23:26:54-4f63e0a/dragonfly-amd64-5_8
2019-11-02T21:51:09-61fa798/plan9-amd64-9front
2019-10-04T20:18:48-86ea7d5/openbsd-amd64-64

--- FAIL: TestServerKeepAlivesEnabled_h2 (0.08s)
    serve_test.go:5689: got same connection between first and second requests
FAIL
FAIL	net/http	5.051s

2019-09-18T18:43:44-45873a2/plan9-386-0intro
2019-09-10T14:25:46-f668573/openbsd-386-64
2019-08-28T05:11:20-07f0460/dragonfly-amd64

See previously #21724.

CC @neild @tombergan

[bcmills]

greplogs --dashboard -md -l -e '(?ms)TestServerKeepAlivesEnabled_h.*got same connection' --since=2021-06-11

2021-11-02T03:09:01-a45457d/illumos-amd64
2021-09-28T17:50:53-8cf0a08/dragonfly-amd64

[gopherbot]

Change https://golang.org/cl/360854 mentions this issue: net/http: deflake TestServerKeepAlivesEnabled_h{1,2}