Move slowTest and externalTest to test/common.js

These are used outside of the integration tests, but including the
integration index file causes the very slow setup of the contorller
and host.  Avoid the integration setup costs when running commands
such as `pnpm mocha test/lib` by moving these utility functions to
the common test functions.

test/integration/instance.js

@@ -3,9 +3,9 @@ import assert from "node:assert/strict";
 import * as lib from "@clusterio/lib";
 import fs from "node:fs/promises";
 
-import { testMatrix } from "../common.js";
+import { slowTest, testMatrix } from "../common.js";
 import {
-	slowTest, exec, execCtl, execCtlProcess, sendRcon, getControl,
+	execCtl, execCtlProcess, sendRcon, getControl,
 	requiresFactorio, hasFactorio, instancesDir,
 } from "./index.js";