net/http: TestTransportDiscardsUnneededConns failures with `use of closed network connection`

[gopherbot]

#!watchflakes
post <- pkg == "net/http" && test == "TestTransportDiscardsUnneededConns" && `use of closed network connection`

Issue created automatically to collect these failures.

Example (log):

--- FAIL: TestTransportDiscardsUnneededConns (0.00s)
    --- FAIL: TestTransportDiscardsUnneededConns/h2 (2.54s)
        clientserver_test.go:1141: 10 connections opened, 8 closed; want 9 to close
        clientserver_test.go:210: server log: http: TLS handshake error from 127.0.0.1:51208: read tcp 127.0.0.1:51199->127.0.0.1:51208: use of closed network connection

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "net/http" && test == "TestTransportDiscardsUnneededConns"

2023-12-01 19:20 darwin-amd64-13 go@5a2161ce net/http.TestTransportDiscardsUnneededConns (log)

--- FAIL: TestTransportDiscardsUnneededConns (0.00s)
    --- FAIL: TestTransportDiscardsUnneededConns/h2 (2.54s)
        clientserver_test.go:1141: 10 connections opened, 8 closed; want 9 to close
        clientserver_test.go:210: server log: http: TLS handshake error from 127.0.0.1:51208: read tcp 127.0.0.1:51199->127.0.0.1:51208: use of closed network connection

— watchflakes

[bcmills]

(attn @neild)
See previously:

• net/http: flaky tests on macOS #33585
• net/http: occasional TestTransportDiscardsUnneededConns flakes with "connection reset by peer" on netbsd-amd64-8_0 builder #36797