invariant: COMBINING ... BELOW normally has ccc=220

[markusicu]

We see some errors or typos in character proposals listing ccc values, sometimes proposing ccc=230 for marks-below. An invariant test could help catch those, if the character names follow the pattern COMBINING ... BELOW.

[roozbehp]

I would change that to anything with gc=Mn and BELOW at the end of the name. Some combining marks, like most of the ones in the Arabic block, don't have COMBINING in the name.