title: run all webdriver.io spec files in single browser session
body: <p>I'm using <code>wdio</code> to run tests on my local machine; I start up a single server and run tests on it.  I reduced <code>maxInstances</code> to 1 so that there are no problems resetting the database before each test.</p>
  
  <p>My config is:</p>
  
  <pre><code>exports.config = {
    specs: [
      './test/wdioTests.js',
    ],
    maxInstances: 1,
    capabilities: [{
      maxInstances: 1,
      browserName: 'phantomjs',
    }],
    sync: true,
    logLevel: 'command',
    coloredLogs: true,
    screenshotPath: './errorShots/',
    baseUrl: process.env.ROOT_URL,
    waitforTimeout: 10000,
    connectionRetryTimeout: 90000,
    connectionRetryCount: 3,
    services: [
      'phantomjs'
    ],
    framework: 'mocha',
    reporters: ['spec'],
    mochaOpts: {
      ui: 'bdd',
      compilers: ['js:babel-core/register'],
    },
  }
  </code></pre>
  
  <p>But <code>wdio</code> logging indicates that it creates a new session before each spec file.</p>
  
  <p>What's worse, if I use <code>--mochaOpts.grep</code>, it still goes through the process of creating a new session for each spec file, even if no tests in that file actually run, which is needlessly time-consuming.</p>
  
  <p>How can I make <code>wdio</code> start one session at the beginning, and not create new sessions before running tests for each spec file?</p>
  
  <p>I'm looking for a better way than just having <code>specs: ['./test/allTests.js']</code> and manually importing all the different spec files in <code>allTests.js</code>.</p>