net/http: TestDontCacheBrokenHTTP2Conn flakes

[josharian]

Several instances on the dashboard. One example:

https://build.golang.org/log/1aa3681cf8816e6e95b24dddb3a9a47922fbc235

cc @bcmills @matloob although I don’t see how either of your changes could have caused this

[cuonglm]

Duplicated of #35113

There’s CL under review