Make pack-n-play errors from system-test easier to debug

[igorbernstein2]

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Currently if there is a pack-n-play the system-tests just fail with the message "Process xxx exited with code y", this doesn't really help debugging the underlying problem. Bubbling up the output from tsc would be a lot more helpful. It seems like the output is recorded in err.stdout: https://github.com/google/pack-n-play/blob/v1.0.0-2/src/pack-n-test.ts#L112, but there is no obvious way to see that information from npm run system-tests. If there is a way, it should be documented, otherwise the build should be changed to make it possible