docs: auto relay example (#795)

* chore: auto relay example

* chore: update examples to use process arguments

* chore: add test setup for node tests and test for auto-relay

* chore: apply suggestions from code review

* chore: do not use promise for multiaddrs event on example

examples/test.js

@@ -0,0 +1,95 @@
+'use strict'
+
+process.env.NODE_ENV = 'test'
+process.env.CI = true // needed for some "clever" build tools
+
+const fs = require('fs-extra')
+const path = require('path')
+const execa = require('execa')
+const dir = path.join(__dirname, process.argv[2])
+
+testExample(dir)
+  .then(() => {}, (err) => {
+    if (err.exitCode) {
+      process.exit(err.exitCode)
+    }
+
+    console.error(err)
+    process.exit(1)
+  })
+
+async function testExample (dir) {
+  await installDeps(dir)
+  await build(dir)
+  await runTest(dir)
+  // TODO: add browser test setup
+}
+
+async function installDeps (dir) {
+  if (!fs.existsSync(path.join(dir, 'package.json'))) {
+    console.info('Nothing to install in', dir)
+    return
+  }
+
+  if (fs.existsSync(path.join(dir, 'node_modules'))) {
+    console.info('Dependencies already installed in', dir)
+    return
+  }
+
+  const proc = execa.command('npm install', {
+    cwd: dir
+  })
+  proc.all.on('data', (data) => {
+    process.stdout.write(data)
+  })
+
+  await proc
+}
+
+async function build (dir) {
+  const pkgJson = path.join(dir, 'package.json')
+
+  if (!fs.existsSync(pkgJson)) {
+    console.info('Nothing to build in', dir)
+    return
+  }
+
+  const pkg = require(pkgJson)
+  let build
+
+  if (pkg.scripts.bundle) {
+    build = 'bundle'
+  }
+
+  if (pkg.scripts.build) {
+    build = 'build'
+  }
+
+  if (!build) {
+    console.info('No "build" or "bundle" script in', pkgJson)
+    return
+  }
+
+  const proc = execa('npm', ['run', build], {
+    cwd: dir
+  })
+  proc.all.on('data', (data) => {
+    process.stdout.write(data)
+  })
+
+  await proc
+}
+
+async function runTest (dir) {
+  console.info('Running node tests in', dir)
+  const testFile = path.join(dir, 'test.js')
+
+  if (!fs.existsSync(testFile)) {
+    console.info('Nothing to test in', dir)
+    return
+  }
+
+  const runTest = require(testFile)
+
+  await runTest()
+}