file already closed error for cli reading logs from go test

An error like ` lvl=eror msg="error reading output: read |0: file already closed"` was reported which seems to be from https://github.com/GoogleCloudPlatform/cloud-foundation-toolkit/blob/93e84cf114487d243b2fde0a2a591ca71f5e848b/cli/bptest/run.go#L90

I think in some edge case of cmd termination there could be a synchronization issue between the logging go routine and waiting for the cmd.