By Joshua Franklin and Aparajita Saxena (Reuters) - Ride-hailing company Lyft Inc beat bigger rival Uber Technologies in filing for an initial public offering (IPO) on Thursday, defying the market jitters that threaten to upset a string of technology unicorns from going public next year. Lyft's IPO will be a big test of stock market investors' appetite for companies that rely heavily on part-time workers. Much of its future hinges on it ability to replace drivers who move on after a few weeks or months when they find better paying jobs.