Make children processes (bedrock servers) die when authtest dies

[heekinho]

Details

Currently, whenever authtest crashes or is killed, it might left behind hanging bedrock servers which brings all sort of weird errors on next runs. Since authtest itself is the one that spawn those servers (fork/exec), let's instruct kernel do make those processes die when authtest dies.

Fixed Issues

Fixes https://github.com/Expensify/Expensify/issues/582719

Tests

I've made a small script to test this:

#!/bin/sh

# Start by killing everything
vssh pkill -9 bedrock
vssh pkill -9 authtest

# Let's simulate a crash and see how many bedrocks are left behind
vssh ./authtest -threads 12 -only "Billing.*" &

sleep 1

# Kill it!
vssh pkill -9 authtest

# Count how many processes match "my_app"
echo "Number of bedrock instances: "
vssh pgrep -c "bedrock"

# Now kill the bedrock instances
vssh pkill -9 bedrock

Before this PR I get several hanging bedrock processes:

Number of bedrock instances: 32

After this PR I get no hanging bedrock:

Number of bedrock instances: 0

---

Internal Testing Reminder: when changing bedrock, please compile auth against your new changes

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