Add test pass on CI with linting enabled

[szymon-rd]

To prevent compiler crashes from happening, we should test all compiler flags on a complex and large set of code snippets. A good candidate for that is the pos test suite. However, to not make the local tests any longer, they should run only on CI.

[nicolasstucki]

How slow are the linting checks? How much time would we add to running the pos tests with these additional flags on?

The previous issue we had with performance came from executing the pos tests twice. Once with the flags and once without. It took roughly twice the time because we ran the entire compilation pipeline again. Enabling them on the main pos tests should only add an additional fraction of testing time. I would assume that linting is faster than compiling. This is a cost that might be fine for local testing as well.

testCompilation should behave the same on the CI or locally, otherwise, we might end up with failures that we cannot easily reproduce locally.

[szymon-rd]

The only problem is the noise in output from warns. However, we could run pos tests with all linting flags locally and enable silent warnings (nowarn).

[nicolasstucki]

When running pos compilation tests (with testCompilation) we never see or check the warnings. I believe we can just add the flags here and we will not notice the change.

[szymon-rd]

A lot of pos tests have Werror enabled. We could create a whitelist for those that we want to run with linting, and run the rest in a separate pass (so split pos tests in two).

[som-snytt]

Perhaps the number of ugly pos tests that necessarily trip a lint warning is small. They can get

//> using option -Xlint:false

or whatever the option is for turning off lints.

(Manual bookkeeping is a dreadful maintenance burden; I think vulpix's programmatic model was weak on that score.)