update test results incrementally instead of wiping out existing results

[michaelficarra]

Currently, as the tests are run, test262.fyi shows only results from engines that have completed. This makes the most recent test results for engines that have not yet completed (or failed?) unavailable until they succeed. It'd be better to just keep the latest results we have and incrementally update them as each engine's results come in. This would make the website continuously useful throughout the update process.