x/net/http2: TestUnreadFlowControlReturned_Server failures due to INTERNAL_ERROR on plan9-386-0intro since 2022-03-15

[bcmills]

--- FAIL: TestUnreadFlowControlReturned_Server (16.39s)
    --- FAIL: TestUnreadFlowControlReturned_Server/body-closed (14.15s)
        server_test.go:3844: body-closed timedout
        server_test.go:3868: body-closed stream error: stream ID 5; INTERNAL_ERROR; received from peer
FAIL
FAIL	golang.org/x/net/http2	184.809s

greplogs --dashboard -md -l -e 'FAIL: TestUnreadFlowControlReturned_.*' --since=2022-01-01

2022-03-30T02:16:17-de3da57-d1060d8/plan9-386-0intro
2022-03-25T01:24:44-27dd868-8ab42a9/plan9-386-0intro
2022-03-18T19:32:40-27dd868-a682a5c/plan9-386-0intro
2022-03-18T16:57:07-27dd868-0a49f70/plan9-386-0intro
2022-03-15T01:00:36-27dd868-44a0da4/plan9-386-0intro

(attn @0intro; CC @neild)

See previously #49645.