Re: Of course they're testing it again
In SpaceX's favour, their engines are far simpler than the Space Shuttle's RS-25s, and are designed to be fired many more times and for far longer than necessary for a single launch. The Falcon 9 first stage also doesn't return from orbit and thus doesn't require the Shuttle's fragile and expensive heat shielding. Etc. etc.
They're two entirely different vehicles, and SpaceX has undoubtedly benefited from the lessons learned by their predecessors.