tests: add test for new rrdcreate feature

Thanks for merging the feature.  I was too slow at cleaning up my test harness and integrating it into your framework, but if you'd like it it's here:

    OK: diagnose zero step : OK: step size: value must be positive
    OK: diagnose negative step : OK: step size: value must be (suffixed) positive number
    OK: diagnose non-text step : OK: step size: value must be (suffixed) positive number
    OK: diagnose misaligned RRA step : OK: Invalid step 15s: value would truncate when scaled
    OK: diagnose misaligned RRA rows : OK: Invalid row count 75s: value would truncate when scaled
    OK: diagnose zero RRA rows : OK: Invalid row count 10s: value would truncate when scaled
    OK: diagnose missing heartbeat separator : OK: failed to parse data source 5: missing separator
    OK: diagnose missing heartbeat : OK: failed to parse data source xx:U:U: value must be (suffixed) positive number
    OK: diagnose max/min error : OK: failed to parse data source 30s:5:3: min must be less than max in DS definition
    OK: diagnose missing max : OK: failed to parse data source 30s:53: failed to extract min:max
    OK: 1-sec PDP equivalents
    OK: 5-sec PDP equivalents
    OK: HWPREDICT equivalents