package/ci: test against Doxygen 1.8.16 and 1.8.18 as well.

package/ci: test against Doxygen 1.8.16 and 1.8.18 as well.

Preparing for the final boss which is 1.9, but!

 * 1.8.13 downloads and runs fine, but there's way too many test
   failures and I'm not ready for these. Probably will just drop the
   support eventually.
 * 1.8.14 to 1.8.15 links to libclang.so.6, which is not a thing on
   Ubuntu 18.14. I suppose this is because SOMEBODY built the damn
   release binary on their own PC, instead of in an old enough sandboxed
   environment, HA?!
 * 1.8.16 and 1.8.17 work, just a bunch of fixes needed to make test
   pass on .16.
 * 1.8.18 works, except it relies on some silly libtinfo.so.5 that I had
   to install.