Add regression tests

[felixhekhorn]

• this would be an alternative to Add LHA aN3LO to ekomark #397 and Add pol NNLO benchmark to ekomark #411
• the strategy will be: search in the (dedicated) DB for given theory+op+PDF and return "eko" column
• data can be generated with external = "void" - which will be available with Rustify QED #416