First of all I think that environment variables are the right tool for the job, although I'd call them TEST_INTERACTIVE or something like that. I'd also provide the ability to provide all information via a config file or environment variables, because the value of a test suite is that you can run it with very little effort - which gets lost if you have to enter the same things all over again.

Does the "export" syntax even work on non-*NIX-shells?

It doesn't even work on some Unix shells, for example on csh.

You could just say "set the environment MyModOPTRUN variable to yes", and hope that the user know how to do it.