There is no such capability at present, but I'm interested in the question.
When developing CodeRunner I assumed students would have a normal IDE at their disposal. They would develop and test their code and then, when satisfied, they would submit it to CodeRunner for grading. If it failed, they would be allowed to resubmit, but would pay a small penalty for doing so. One of the themes that emerged was the idea of "authentic assessment" - students would have normal development tools at their disposal and the product they produced would then be assessed via CodeRunner.
It sounds like you expect students to develop and debug their code within CodeRunner. Could you explain why you do not let them have access to normal programming development tools please? Or perhaps to a web IDE like https://repl.it ?
Putting that question to one side and accepting that for simple problems students might wish to enter code directly into CodeRunner, can you clarify for me how you would expect students to test their code? You say their stdins would be independent from the testcases. What about the test code within the testcases? Would that be ignored too? And what about all the code in the template, that wraps the student code to make a full executable, and which often includes extra checks on style as well?
Thanks for raising the question. I've mulled over the idea of incorporating more development capabilities into CodeRunner before, but I'm not sure it's a good idea. Certainly providing a full web IDE within a Moodle question sounds wrong. A totally separate Moodle web IDE plugin that linked into CodeRunner might in principle be possible but then again why should it be part of Moodle rather than independent, like repl.it?