Well, in theory, Scirra has covered all that ground so that the rest of us don't have to. Their standard format for making plugins- unless you're trying to implement something that's just like, extremely unusual- is very simple and straightforward. So, you wouldn't have to test it under every possible circumstance for the same reasons you wouldn't have to test a new lightbulb in every commercially-available lamp. As long as you know that the lightbulb can physically function and you don't try to use it to paint your living room or something, you should be good to go.
The only exceptions to this that I can think of are very framework-dependent applications. But even then, all you'd have to do is include whatever script-files or libraries your thing depends on inside the project file... Or, in the case that someone is wanting to develop for a platform that isn't capable of running JS programs, it's a pretty safe bet that Construct is the wrong tool. Apparently there's also some minification issues, but that's not technically necessary, and although inconvenient, doesn't restrict what you can actually do.