x/net/http2: TestTransportBody* flaky on android and ppc64

[bcmills]

The x/net/http2.TestTransportBody tests seem to flake with request body larger than specified content length errors at a relatively high rate on certain builders.

Examples:

• android-386-emu:
  • https://build.golang.org/log/d984e27183b94c7e368d94218884779974344556
  • https://build.golang.org/log/1f7126e68b628480fe5a654699de8824eeca44f4
• linux-ppc64-buildlet
  • https://build.golang.org/log/96c9c2893db745808b15290db8d7ad800bd5b773
  • https://build.golang.org/log/dda7e3f43e4ae4a8f29654ff93a82772bd6db199
  • https://build.golang.org/log/638ae27a8305403407f12f1c4e55ee3bd8dc1eab
  • https://build.golang.org/log/7772ce82be9e6ce7a9f2a40974c3a2abcdfa2545

CC @bradfitz @tombergan