net/http: occasional TestTransportDiscardsUnneededConns flakes with "connection reset by peer" on netbsd-amd64-8_0 builder

[bcmills]

2020-01-24T19:37:58-67539f6/netbsd-amd64-8_0
2019-10-21T17:23:42-06ac262/netbsd-amd64-8_0
2019-08-29T15:21:31-35ac194/netbsd-amd64-8_0
2019-05-22T04:43:25-648c7b5/netbsd-amd64-8_0

--- FAIL: TestTransportDiscardsUnneededConns (0.04s)
    clientserver_test.go:1012: Get: Get "https://127.0.0.1:56151": dial tcp 127.0.0.1:56151: connect: connection reset by peer
FAIL
FAIL	net/http	9.694s

Note that this test is also flaky (with a somewhat different failure mode) on macOS — see #33585.

CC @bradfitz @bsiegert @toothrot @dmitshur @cagedmantis

[bcmills]

2020-03-09T19:17:27-2818a11/netbsd-amd64-8_0

[bcmills]

2020-03-14T07:03:15-d774d97/netbsd-amd64-8_0

[bsiegert]

Thanks!

/cc @tklauser