Implement rules testing a la promtool test rules <test file>.yml

[genebean]

The fix for #148 does not account for promtool test rules <test file>.yml. Please also fix that since the namespace bits break the upstream tool.

[freddd]

+1, we recently upgraded promtool and was hit with the same issue. This PR: https://github.com/prometheus/prometheus/pull/7767 was the one that broke promtool for us since it's now doing strict validation and not allowing namespace.