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/utils.js

@@ -0,0 +1,61 @@
+'use strict'
+
+const execa = require('execa')
+const fs = require('fs-extra')
+const which = require('which')
+
+async function isExecutable (command) {
+  try {
+    await fs.access(command, fs.constants.X_OK)
+
+    return true
+  } catch (err) {
+    if (err.code === 'ENOENT') {
+      return isExecutable(await which(command))
+    }
+
+    if (err.code === 'EACCES') {
+      return false
+    }
+
+    throw err
+  }
+}
+
+async function waitForOutput (expectedOutput, command, args = [], opts = {}) {
+  if (!await isExecutable(command)) {
+    args.unshift(command)
+    command = 'node'
+  }
+
+  const proc = execa(command, args, opts)
+  let output = ''
+  let time = 120000
+
+  let timeout = setTimeout(() => {
+    throw new Error(`Did not see "${expectedOutput}" in output from "${[command].concat(args).join(' ')}" after ${time/1000}s`)
+  }, time)
+
+  proc.all.on('data', (data) => {
+    process.stdout.write(data)
+
+    output += data.toString('utf8')
+
+    if (output.includes(expectedOutput)) {
+      clearTimeout(timeout)
+      proc.kill()
+    }
+  })
+
+  try {
+    await proc
+  } catch (err) {
+    if (!err.killed) {
+      throw err
+    }
+  }
+}
+
+module.exports = {
+  waitForOutput
+}