Add runner.sh script to start/stop daemons as CTest fixtures

See https://discourse.cmake.org/t/ctest-able-to-daemonize-fixtures/1012/3
for a rationale.

scripts/runner.sh

+#!/usr/bin/env bash
+
+function help() {
+  echo "Usage:"
+  echo "  $(basename "$0") COMMAND"
+  echo ""
+  echo "  Where COMMAND is one of:"
+  echo "    start PIDFILE PROGRAM ARGS...  Run PROGRAM and record its PID in PIDFILE"
+  echo "    stop SIGNAL PIDFILE            Send SIGNAL to the PID contained in PIDFILE"
+  echo "    help                      Print this message"
+}
+
+function readpid() {
+
+  if [ $# -eq 0 ]; then
+    echo "FATAL: readpid(): Missing pifile" >&2
+    exit 1
+  fi
+
+  pidfile="$1"
+
+  read -r pid <"$pidfile"
+  echo $pid
+}
+
+function run() {
+
+  if [ $# -eq 0 ]; then
+    echo "FATAL: missing program pidfile" >&2
+  elif [ $# -eq 1 ]; then
+    echo "FATAL: missing program to run" >&2
+    help
+    exit 1
+  fi
+
+  pidfile="$1"
+  shift
+
+  if [ -e "$pidfile" ]; then
+    pid=$(readpid "$pidfile")
+    echo "$pid"
+
+    if pgrep --pidfile "$pidfile"; then
+      exit 1
+    fi
+  fi
+
+  "$@" 2>/dev/null 1>/dev/null 0</dev/null &
+  pid=$!
+  echo $pid >"$pidfile"
+  sleep 1
+
+  if ! kill -0 $pid; then
+    echo "Process $pid does not seem to exist." >&2
+    echo "The program below may not exist or may have crashed while starting:" >&2
+    echo "  $*" >&2
+    exit 1
+  fi
+
+  exit 0
+}
+
+function stop() {
+
+  if [ $# -eq 0 ]; then
+    echo "FATAL: missing signal" >&2
+    exit 1
+  elif [ $# -eq 1 ]; then
+    echo "FATAL: missing pidfile" >&2
+    exit 1
+  fi
+
+  signal="$1"
+  pidfile="$2"
+
+  if [ ! -e "$pidfile" ]; then
+    echo "FATAL: pidfile '$pidfile' does not exist" >&2
+    exit 1
+  fi
+
+  if pkill "-$signal" --pidfile "$pidfile"; then
+    rm "$pidfile"
+    exit 0
+  fi
+  exit 1
+}
+
+if [ $# -eq 0 ]; then
+  echo "FATAL: missing arguments" >&2
+  help
+  exit 1
+fi
+
+case $1 in
+
+start)
+  shift
+  run "$@"
+  ;;
+
+stop)
+  shift
+  stop "$@"
+  ;;
+
+help)
+  help
+  exit 0
+  ;;
+esac