Add smoke test with engine using Docker

.test/minetest.conf

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+mg_name = singlenode
+mesecon.internal_test = true
+random_mod_load_order = true

.test/run.sh

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+#!/bin/bash
+tempdir=$(mktemp -d)
+confpath=$tempdir/minetest.conf
+worldpath=$tempdir/world
+trap 'rm -rf "$tempdir" || :' EXIT
+
+[ -f mesecons/mod.conf ] || { echo "Must be run in modpack root folder." >&2; exit 1; }
+
+command -v docker >/dev/null || { echo "Docker is not installed." >&2; exit 1; }
+mtg=.test/minetest_game
+[ -d $mtg ] || echo "A source checkout of minetest_game was not found. This can fail if your docker image does not ship a game." >&2
+
+mkdir "$worldpath"
+cp -v .test/minetest.conf "$confpath"
+chmod -R 777 "$tempdir"
+
+args=(
+	-v "$confpath":/etc/minetest/minetest.conf
+	-v "$tempdir":/var/lib/minetest/.minetest
+	-v "$PWD":/var/lib/minetest/.minetest/world/worldmods/mesecons
+)
+[ -d $mtg ] && args+=(
+	-v "$(realpath $mtg)":/var/lib/minetest/.minetest/games/minetest_game
+)
+args+=("$DOCKER_IMAGE")
+[ -d $mtg ] && args+=(--gameid minetest)
+docker run --rm -i "${args[@]}"
+
+ls -la "$worldpath"
+test -f "$worldpath/mesecon_actionqueue" || exit 1
+exit 0