A0-4324: Alter test_db_sync.sh to kill aleph-node after tests (#1817)

# Description

Alters test_db_sync.sh script to kill aleph-node process once things are
tested. It seems that cleaning github runner workspace, which is a next
step that executes bunch of `rm -rf` commands, is failing because
aleph-node running in the background keeps creating new files in the
directories that are meant to be removed (hence, the `rm -rf` commands
fail with `Directory not empty`).

.github/scripts/test_db_sync.sh

@@ -138,6 +138,7 @@ chmod +x aleph-node
     --node-key-file "${BASE_PATH}/p2p_secret" \
     ${DB_ARG[*]} \
     --no-mdns 1>/dev/null 2> "${BASE_PATH}/aleph-node.log" &
+ALEPH_NODE_PID=$!
 
 get_current_block
 echo "Syncing to ${TARGET_BLOCK} starting at ${CURRENT_BLOCK}."
@@ -154,3 +155,5 @@ if [[ "${MARK_SNAPSHOT_AS_LATEST}" == "true" ]]; then
       --website-redirect "${S3_URL}/${SNAPSHOT_DAY}/${S3_SNAPSHOT_PREFIX}_${SNAPSHOT_DAY}.tar.gz"
 fi
 
+kill -9 $ALEPH_NODE_PID
+