x/net/http2: TestTransportReqBodyAfterResponse_403 failures with "want (0,10485760) exclusive" on js-wasm since Oct 4

[bcmills]

--- FAIL: TestTransportReqBodyAfterResponse_403 (0.10s)
    transport_test.go:837: client: For 403 response, Transport wrote 10485760 bytes; want (0,10485760) exclusive
FAIL
FAIL	golang.org/x/net/http2	69.113s

The first occurrence of this failure mode in the logs was at CL 353390 (CC @neild), so this looks like a recent regression.

greplogs --dashboard -md -l -e '(?ms)FAIL: TestTransportReqBodyAfterResponse_403.*want .* exclusive'

2021-10-05T07:32:41-d4b1ae0-d55009c/js-wasm
2021-10-04T19:50:52-b30845b-7d822af/js-wasm
2021-10-04T18:58:42-cedda3a-7d822af/js-wasm
2021-10-04T16:44:53-cedda3a-9432320/js-wasm

[gopherbot]

Change https://golang.org/cl/354130 mentions this issue: http2: avoid race in TestTransportReqBodyAfterResponse_403.