Use `set -e` and `trap` to improve test shell-script

[mpolitzer]

Context

What problem are you trying to solve?

Code improvement

As per: #171 (review)

We could automate cleanup and exit on any error for scripts: tests/scripts/test-jsonrpc-server.sh and tests/scripts/test-grpc-server.sh.

with:

set -e
trap wait_for_shutdown EXIT

note: here docs return false for whatever reason, use this trick to make them work.

cat << EOF | true
EOF