it's a fair question you are asking. I have some mechanical systems but I never follow every trade and I admit that. I don't like automated trading because for me trading is fun and I want to do it. I also like to trade whenever I feel like, not when the systems issue signals. I use Multicharts and a price pattern scanner that comes with a software called Price Action Lab. This program identifies price patterns based on my criteria and it has a portfolio backtesting feature you can use to see how they have done on a large group of symbols and this gets you a large sample of trades. I trade high cap stocks and I backtest the price patterns on historical data from 500 stocks to see how they have performed. I want to see more than 5,000 trades on the portfolio and a profit factor greater than 1.3 with at least 280 of the symbols winners. I then use code that this program generates to test the patterns in Multicharts with a 100-ma and RSI (14) filter. If price is above the 100-ma and RSI(14) is less than 50 but greater than 30 I apply another filter (my secret). This requires some work but it's fun. Most of the hard part is automated though. It takes a few minutes to scan all the tickers I follow after the market close, the portfolio backtest is done in less than a minute and my own analysis takes about an hours. I hope I answered your question.
Why would anyone want to backtest in a public platform instead of getting something like Amibroker and doing it for own benefit? Unless I am missing something.
I use this site for my analysis needs. It is still in beta but it can do some cool stuff other web based tools can't do. It's free. http://fasterbull.com/ fan27
Backtesting online is ta proliferated service now. I expect to get paid soon to visit websites so they can get my inputs.
If you use R you can have a look at http://quantstrattrader.wordpress.com The blogger backtests quantitative strategies from different sources in R (code published) and gives great analysis of the results backed by statistical metrics.