net/http: TestTimeoutHandlerContextCanceled failures beginning 2021-11-06

[bcmills]

--- FAIL: TestTimeoutHandlerContextCanceled (0.00s)
    serve_test.go:2551: got unexpected Write error on first request: <nil>
FAIL
FAIL	net/http	53.045s

greplogs --dashboard -md -l -e 'FAIL: TestTimeoutHandlerContextCanceled'

2021-11-08T16:44:14-7ee3f14/linux-386-longtest
2021-11-08T16:44:14-7ee3f14/linux-amd64-jessie
2021-11-07T04:57:22-9e6ad46/linux-386-longtest
2021-11-06T19:41:15-61d789d/freebsd-amd64-race
2021-11-06T19:41:14-cfb3dc7/linux-amd64-longtest

(CC @neild)

[gopherbot]

Change https://golang.org/cl/361920 mentions this issue: net/http: deflake TestTimeoutHandlerContextCanceled