plugin: Add basic loading test

Will work on testing overlay next.

test/plugin.test.ts

@@ -0,0 +1,31 @@
+import { describe } from "mocha"
+import { PluginAPI } from "../src/node/plugin"
+import { logger } from "@coder/logger"
+import * as path from "path"
+import * as assert from "assert"
+
+/**
+ * Use $LOG_LEVEL=debug to see debug logs.
+ */
+describe("plugin", () => {
+  it("loads", async () => {
+    const papi = new PluginAPI(logger, path.resolve(__dirname, "test-plugin") + ":meow")
+    await papi.loadPlugins()
+
+    // We remove the function fields from the application's plugins.
+    const apps = JSON.parse(JSON.stringify(await papi.applications()))
+
+    assert.deepEqual([
+      {
+        name: "goland",
+        version: "4.0.0",
+        iconPath: "icon.svg",
+        plugin: {
+          name: "test-plugin",
+          version: "1.0.0",
+          description: "Fake plugin for testing code-server's plugin API",
+        },
+      },
+    ], apps)
+  })
+})