Use transport_version() test base class method instead of variants like connection_->transport_versions() and GetParams().transport_version. Change transport_version() implementation so that it can be called at any time, including before connection_ is initialized. gfe-relnote: n/a (test-only change) PiperOrigin-RevId: 300659212 Change-Id: I5cc111d045b8e744735425d0b7f9072849d4e5b8