Make sure we wait for stdout/stderr pipes to be closed before calling Wait()

[gabivlj]

Fixes #298

[gabivlj]

Hi @tomhjp! Could you take a look at these changes whenever you have a chance? Thank you!!

[tomhjp]

Thanks for the PR and the ping! I'd like to get a test for this fix, please could you share details on a repro? I can reliably repro if similarly to the previous issue I add a time.Sleep(time.Second) inside the stdout scanner loop, but I want to be sure that that's a faithful representation of the real scenario, i.e. is this a race condition that only pops up once in a while, or does it happen every time for your plugin?

[tomhjp]

I pushed a test that fails before this fix and passes after in 36c021d. LMK if you think that looks like a good test case.

[gabivlj]

Thank you @tomhjp for the test! I appreciate it a lot.
I think it's a good test case, as it will practically trigger that stderr exits before stdout and call the 'Wait()' syscall before stdout goroutine finishes.

We were encountering these logs "sometimes" in our error reports so it was just noise.

[tomhjp]

Awesome, thanks for the confirmation! In that case I'm convinced we've understood the bug sufficiently well to merge. I'll add the test in a follow up PR. Thanks again for the fix!