Add warning when using generators that produce more than one test case in a run

[cannor147]

What is the reason for warning these situations? Many people use multiple generators for generating tests of different kinds. I think, it's normal way to use more than one generators.

[KalininN]

I mean exactly the opposite (edited issue name). My point (and everyone I asked agree) that the gen >{1-100} syntax is much harder to support: you can't use them in stresses, you can't reorder tests, remove/add from web interface, you can't view parameters without looking into code.

[MikeMirzayanov]

Done