x/tools/cmd/godoc uses the usually-test-only net/http/httptest package.

But because httptest assumes it's running in a test, it registers its own flags, which means godoc ends up with mysterious flags:

  -goroot string
        Go root directory (default "/home/bradfitz/go")
  -http string
        HTTP service address (default "localhost:6060")
  -httptest.serve string
        if non-empty, httptest.NewServer serves on this address and blocks
  -index
        enable search index

Either stop using in httptest in godoc, or change httptest to somehow conditionally register that flag. Perhaps only if it looks like we're in a test binary. (check lookup of other testing flags?)