spin-off from #1298: add tests for options with negative values

It became apparent while working on #1298 (closed) that options can have negative values. Specifically the ones with OPT_INT[0-9]+_TYPE as opposed to OPT_UINT[0-9]+_TYPE.

And it became apparent that we don't have any tests validating that the value is parsed correctly, without errors, etc.

Let's add some.