function loadTest(options, callback) {
	if (!callback) {
		return loadTestAsync(options)
	}
	loadTestAsync(options).then(result => callback(null, result)).catch(error => callback(error))
}
Run a load test.
Parameters:
- `options`: an object which may have:
	 - url: URL to access (mandatory).
	 - concurrency: how many concurrent clients to use.
	 - maxRequests: how many requests to send
	 - maxSeconds: how long to run the tests.
	 - cookies: a string or an array of strings, each with name:value.
	 - headers: a map with headers: {key1: value1, key2: value2}.
	 - method: the method to use: POST, PUT. Default: GET, what else.
	 - data: the contents to send along a POST or PUT request.
	 - contentType: the MIME type to use for the body, default text/plain.
	 - requestsPerSecond: how many requests per second to send.
	 - agentKeepAlive: if true, then use connection keep-alive.
	 - indexParam: string to replace with a unique index.
	 - insecure: allow https using self-signed certs.
	 - secureProtocol: TLS/SSL secure protocol method to use.
	 - proxy: use a proxy for requests e.g. http://localhost:8080.
	 - quiet: do not log any messages.
	 - debug: show debug messages (deprecated).
	 - requestGenerator: use a custom function to generate requests.
	 - statusCallback: function called after every request.
	 - tcp: use TCP sockets (experimental).
- `callback`: optional `function(result, error)` called if/when the test finishes;
if not present a promise is returned.