async function onlyTestAtOrAbove(semver, ctx) {
  const versions = {
    3.4: [3, 4],
    3.6: [3, 6]
  };

  if (semver.length !== 3 || Object.keys(versions).indexOf(semver) === -1) {
    throw new TypeError('onlyTestAtOrAbove expects either ' + Object.keys(versions).join(', ') + ' as first parameter.');
  }

  const version = await start.mongodVersion();

  const desired = versions[semver];

  const meetsMinimum = version[0] > desired[0] || (version[0] === desired[0] && version[1] >= desired[1]);

  if (!meetsMinimum) {
    ctx.skip();
  }
}
Helper function to test operators that only work in a specific version of MongoDB and above (such as some aggregation pipeline operators)

@param {String} semver, `3.4`, specify minimum compatible mongod version
@param {Object} ctx, `this`, so that mocha tests can be skipped
@return {Promise<void>}