net/http: TestServerSetKeepAlivesEnabledClosesConns is flaky

[bcmills]

Seen on release-branch.go1.13, but I'm not aware of any changes so far in 1.14 that would have addressed it.

On the aix-ppc64 builder (https://build.golang.org/log/2f6288cba5f841204de6c1761093e54fa59e3839):

--- FAIL: TestServerSetKeepAlivesEnabledClosesConns (2.15s)
    serve_test.go:5536: idle count after SetKeepAlivesEnabled called = 1; want 0
FAIL
FAIL	net/http	3.725s

See previously #17754 (CC @mdempsky @bradfitz)

[bcmills]

A couple of different failure modes. aix and plan9 tend to be

    serve_test.go:5541: idle count after SetKeepAlivesEnabled called = 1; want 0

while the others tend to be

    serve_test.go:5520: expected first two requests on same connection

It's not obvious to me whether those two failure modes share an underlying cause.

2019-12-06T00:57:11-7673884/aix-ppc64
2019-12-04T19:27:51-98e7270/plan9-arm
2019-11-23T00:06:41-f29e53d/aix-ppc64
2019-11-13T21:24:35-718f553/plan9-arm
2019-11-11T19:56:33-0e312f2/aix-ppc64
2019-11-11T17:59:46-6375fe4/freebsd-amd64-race
2019-11-05T00:00:43-ea0b4e7/plan9-arm
2019-10-17T21:10:22-f922645/openbsd-386-62
2019-10-10T11:31:54-900ebcf/plan9-arm
2019-09-03T19:33:04-d2f958d/plan9-arm
2019-08-29T15:21:31-35ac194/plan9-arm
2019-08-28T16:28:19-8419844/illumos-amd64-joyent
2019-08-08T20:38:47-1dc0110/aix-ppc64

[bcmills]

2020-01-24T17:36:44-1f9f88b/aix-ppc64
2019-12-09T20:19:32-7d24e95/aix-ppc64

[bcmills]

2020-03-11T18:47:13-eafb4d8/aix-ppc64
2020-03-11T16:49:13-cf82fea/aix-ppc64
2020-02-27T20:32:29-4ae1879/plan9-arm

[bcmills]

2020-04-06T15:22:07-9853481/aix-ppc64
2020-04-03T00:09:20-041bcb3/aix-ppc64
2020-04-01T03:54:44-afc480b/plan9-arm
2020-03-31T13:13:58-a3d8c21/plan9-arm
2020-03-18T17:29:01-6197104/aix-ppc64

[bcmills]

2020-12-11T22:13:50-41d8e61/aix-ppc64
2020-12-10T16:38:15-56b783a/aix-ppc64
2020-12-03T16:05:09-5246fa5/aix-ppc64
2020-11-05T16:46:56-04b5b4f/aix-ppc64
2020-11-01T22:43:55-333d201/aix-ppc64
2020-10-15T18:30:07-64fb6ae/aix-ppc64
2020-09-08T18:52:38-1e6ad65/aix-ppc64
2020-05-21T14:48:43-39ea0ea/aix-ppc64
2020-05-11T15:33:30-daf39d0/plan9-arm
2020-05-08T13:29:11-b1a48af/aix-ppc64
2020-05-07T23:41:27-5c13cab/aix-ppc64
2020-05-07T20:18:02-3321303/plan9-arm
2020-04-29T21:29:08-b3863fb/plan9-arm
2020-04-29T11:48:24-7250dd2/plan9-arm
2020-04-27T15:53:46-9b9556f/plan9-arm
2020-04-22T20:11:06-ab7a65f/aix-ppc64
2020-04-08T15:12:49-7a35d39/plan9-arm