Adding more projects to the Project Tester

[Seb]

So learning from the recent Vibe.d regression fiasco (we 
temporarily disabled a subconfiguration in Vibe.d and promptly 
got a regression in 2.081), I think we should try to add more 
projects to the Project Tester.

The current list is here:
https://github.com/dlang/ci/blob/master/vars/runPipeline.groovy#L443

Any suggestions?

Why should I add my project to the Project Tester?
--------------------------------------------------

Once a project is added to the Project Tester, DMD can't regress 
on it anymore as for every PR at dmd, druntime, phobos, tools and 
dub the testsuite of the added projects are run.

How does the Project Tester work?
---------------------------------

- By default, it will run the same commands as Travis would do. 
Although, if necessary, custom commands can be used too.
- It will checkout the latest, stable git tag

Requirements
------------

- moderately popular or was prone to regressions in the past
- rather easy to build (i.e. you don't need to download and 
recompile clang)
- no flaky testsuite (random errors in the testsuite due to 
network connectivity shouldn't happen. Though there's 
`DETERMINISTIC_HINT=1` set, s.t. you could disable such parts of 
your testsuite)
- reachable author or development (if there's ever a case where 
we need to push changes via a trivial PR to the repo, it 
shouldn't sit in the queue for weeks)