Fix up tests

[xendk]

Add lexical-binding and newer Emacs versions.

[xendk]

Let's see what the runners says to this.

[xendk]

Well, apparently newest flycheck requires Emacs 27, so remove the older versions.

Now it's only the snapshot release that fails.

[doublep]

Do you see what's wrong on the snapshot?

[xendk]

Well, it seems that it can't find dependency-a when checking test/project-a.el.

test/project-a.el requires test-util.el which in turn requires project-a which is the one with the dependency-a dependency.

Could it be that the load-path isn't set up properly with package archives when running on non-main files? I must admit this is a bit magic to me at the moment.

[doublep]

OK, I will investigate when I found time.