Add a github action

[eric-wieser]

This verifies that tests pass on master and in PRs.

[eric-wieser]

(you can see proof this works at https://github.com/eric-wieser/cadical/tree/ci; the actual CI won't run here unless you merge the PR)

[arminbiere]

Thanks, but, at this point we have no plans to do CI on github.

[eric-wieser]

How about CI on GitHub? (Perhaps you have a private gitlab copy I can't see somewhere)

Note that no real plan is needed; merging these 22 lines of config will be sufficient to enable CI; and it it gets in the way somehow, it can be disabled by deleting this file again.

[arminbiere]

Typo: meant 'github' not 'gitlab' (fixed).

I just do not want to generate any expectations here. That is why I prefer not to have any hint at providing CI support in the main branch.

Our main concern is actually not CI in terms of compilation and running unit tests but performance regressions, which for such solvers is not only a major headache in industrial use but also expensive to test (we are talking about thousands of CPU hours per test) and even much worse to debug (our next release will remove some of them). This can not be done with a CI anyhow.