net/http: TestCancelRequestWhenSharingConnection failures with nil err instead of `Canceled`

[gopherbot]

#!watchflakes
post <- pkg == "net/http" && test == "TestCancelRequestWhenSharingConnection" && `got err .*, want Canceled`

Bug automatically created to track these flakes.

— watchflakes

[gopherbot]

Found new matching flaky dashboard failures for:

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

2022-09-03 18:21 windows-arm64-10 go@a0f05823 net/http.TestCancelRequestWhenSharingConnection (log)

--- FAIL: TestCancelRequestWhenSharingConnection (0.00s)
    transport_test.go:6524: request 2: got err <nil>, want Canceled

— watchflakes

[bcmills]

https://storage.googleapis.com/go-build-log/1e6b7c4d/linux-amd64-unified_9f4321a1.log (a linux-amd64-unified TryBot on https://go.dev/cl/445596)

(attn @neild)

[wdvxdr1123]

a trybot failed on https://go.dev/cl/414954 (linux-amd64-race)
https://storage.googleapis.com/go-build-log/c4a4d844/linux-amd64-race_f1e0101f.log

[gopherbot]

Change https://go.dev/cl/446676 mentions this issue: net/http: deflake TestCancelRequestWhenSharingConnection

[bcmills]

https://storage.googleapis.com/go-build-log/20b2f437/windows-386-2012_09808bc8.log (windows-386-2012 TryBot)