One way might be to just run woff2_compress in a test and see if the result is the same as what's in git. A test like this would probably be added to ReleaseCheckListTest. Would need to add woff2 to travis.
woff2 is not available as a package on travis, but it seems it can be built from source (and then added to a travis cache directory), see:
I don't think woff2 tests are needed. Woff2 is used by default by all modern browsers, and all the fonts are generated in one go just like .woff, .ttf, ... when we edit the font. Can we close the issue maybe?
@tsteur Unfortunately they are not generated in one go as icomoon doesn't allow creating woff2 files in the free version.