net/http: TestClientRedirect308NoGetBody failures

[gopherbot]

#!watchflakes
post <- pkg == "net/http" && test == "TestClientRedirect308NoGetBody"

Issue created automatically to collect these failures.

Example (log):

1 second passes in backend, proxygone= false
--- FAIL: TestClientRedirect308NoGetBody (0.00s)
    --- FAIL: TestClientRedirect308NoGetBody/h2 (2.74s)
        clientserver_test.go:210: server log: timeout waiting for SETTINGS frames from 127.0.0.1:60218
        client_test.go:565: Post "https://127.0.0.1:60217": unexpected EOF

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- pkg == "net/http" && test == "TestClientRedirect308NoGetBody"

2023-05-23 04:38 freebsd-riscv64-unmatched go@e4e8f9b8 net/http.TestClientRedirect308NoGetBody (log)

1 second passes in backend, proxygone= false
--- FAIL: TestClientRedirect308NoGetBody (0.00s)
    --- FAIL: TestClientRedirect308NoGetBody/h2 (2.74s)
        clientserver_test.go:210: server log: timeout waiting for SETTINGS frames from 127.0.0.1:60218
        client_test.go:565: Post "https://127.0.0.1:60217": unexpected EOF

— watchflakes

[bcmills]

This looks like maybe just a slow builder + too-short arbitrary timeout?
(https://cs.opensource.google/go/go/+/refs/heads/master:src/net/http/h2_bundle.go;l=3868;drc=5abfdc8c75b13edbe3ccc918df47dd2da85454d8)

(attn @neild @golang/riscv64)