Add openSUSE Tumbleweed to CI

openSUSE Tumbleweed is the openSUSE rolling distribution delivering the very latest bits. Contrary to Fedora and Debian Sid, which are leading edge distribution, Tumbleweed is bleedingedge distro, shipping the very latest, but tested extensively, bits.

Recently, #1298 (closed) was identified by developer by upgrading his environment to Ubuntu 19.10 (Eoan Ermine), the latest non-LTS release.

If we had Tumbleweed in the CI, we would have identified the issue around September 27. Debian Sid with glibc 2.29 is not leading here to a degree some other stable distribution from other vendors are.