Add tests for statistics_exporter plugin

These were already made at the time I ported the plugin but somehow I
forgot to add them to the statistics rework commit (26eefb0) :/.

plugins/statistics_exporter/test/plugin.js

@@ -0,0 +1,53 @@
+const assert = require('assert').strict;
+
+const mock = require('../../../test/mock');
+const instance = require('../instance');
+const info = require('../info');
+
+
+describe("statistics_exporter plugin", function() {
+	describe("class InstancePlugin", function() {
+		let instancePlugin;
+		before(async function() {
+			instancePlugin = new instance.InstancePlugin(info, new mock.MockInstance());
+			await instancePlugin.init();
+		});
+
+		describe(".onMetrics()", function() {
+			it("should record statistics", async function() {
+				instancePlugin.instance.server.rconCommandResults.set(
+					'/sc remote.call("statistics_exporter", "export")',
+					JSON.stringify({
+						game_tick: 100,
+						player_count: 3,
+						force_flow_statistics: {
+							player: {
+								item_production_statistics: {
+									input: { "inserter": 10, },
+									output: { "iron-plate": 24, }
+								},
+							}
+						},
+						game_flow_statistics: {
+							pollution_statistics: {
+								input: { "boiler": 2000 },
+								output: { "tree-proxy": 560 },
+							}
+						},
+					})
+				);
+
+				await instancePlugin.onMetrics();
+
+				assert.equal(instance._instancePlayerCount.labels('7357').get(), 3);
+				assert.equal(instance._instanceGameTicksTotal.labels('7357').get(), 100);
+				assert.equal(instance._instanceForceFlowStatistics.labels(
+					'7357', 'player', 'item_production_statistics', 'input', 'inserter').get(),
+				10);
+				assert.equal(instance._instanceGameFlowStatistics.labels(
+					'7357', 'pollution_statistics', 'input', 'boiler').get(),
+				2000);
+			});
+		});
+	});
+});