Some updates and tweaks to Azure CI pipelines

[MasterDuke17]

The main pipelines now use Ubuntu 20.04 (the latest available) and I added
single 18.04 and 16.04 pipelines for GCC and Clang (so four new in total)
so we still get some minimal testing on older versions.

[patrickbkr]

lgtm

[patrickbkr]

Did we decide to just accept the occasional test failures on Windows caused by the -j2 for now? (This is not meant as a suggestive question. I really am unsure if I've maybe missed a bit of discussion about this.)

[MasterDuke17]

Did we decide to just accept the occasional test failures on Windows caused by the -j2 for now? (This is not meant as a suggestive question. I really am unsure if I've maybe missed a bit of discussion about this.)

I don't think there was any definite yeas or nays. @ugexe said he prefers to only use a single job in his CI pipelines for reproducibility, but I don't believe he actually voted against this. I prefer using multiple jobs as that is more likely to uncover potential problems, even though it may make reproducing them harder. Since there are a couple approves for this PR I'll leave it as is (adding the -j2), but if there's a subsequent consensus against that's fine, we can revert.

In the multiple runs since that first one, I haven't seen that problem re-appear.

[patrickbkr]

Looking good. 👍🏽