Go v1.16 introduced module-aware commands by default. This version
upgrade broke our http2 interop tests (see #25660). Setting the go
environment variable `GO111MODULE` to "auto" makes go tools behave as
they did before the v1.16 change, but will not be compatible with v1.17.
This PR introduces a minimal go.mod file for the interop test to appease go
v1.16.
Docs on the flag: https://golang.org/ref/mod#mod-commands
Recent discussion on golang/go:
https://github.com/golang/go/issues/31997#issuecomment-782864390