We use it to gate code that depends on custom Go toolchain, but it's
currently only passed in the corp runners. Add a set on OSS so that we
can catch regressions earlier.
To specifically test sockstats this required adding a build tag to
explicitly enable them -- they're normally on for iOS, macOS and Android
only, and we don't run tests on those platforms normally.
Signed-off-by: Mihai Parparita <mihai@tailscale.com>
782ccb5655