cmd/containerboot: avoid leaking bash scripts after test runs

The test was sending SIGKILL to containerboot, which results in no
signal propagation down to the bash script that is running as a child
process, thus it leaks.

Minor changes to the test daemon script, so that it cleans up the socket
that it creates on exit, and spawns fewer processes.

Fixes tailscale/corp#14833
Signed-off-by: James Tucker <james@tailscale.com>
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