ARC handler test expected results need update

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996806

The new version of authheaders also provides the A-R comment for ARC results, which changes the expected test values. I'd recommend changing the tests to only compare the first X characters of the result so it works with either authheaders 0.13.0 or 0.14.0.

The Debian test failure isn't with the current version, but I checked master on Gitlab an this is unchanged.