Revert "Revert "Re-enable lts based testing""

This reverts commit 8a8d5f71b1.

There will be a separate GH Actions workflow for rebuilding the manpage,
and generatemanpage will be dropped from xmonad.cabal (see
https://github.com/xmonad/xmonad/issues/283) therefore the revert no
longer makes sense and the stack workflow can indeed be reverted back to
a working state so we can continue from there.

For more information, see the following revenge revert storm:

Related: https://github.com/xmonad/xmonad/issues/283
Related: https://github.com/xmonad/xmonad/pull/260
Related: https://github.com/xmonad/xmonad/pull/261
Related: https://github.com/xmonad/xmonad/pull/265
Related: https://github.com/xmonad/xmonad/pull/266
Related: https://github.com/xmonad/xmonad/pull/267
Related: https://github.com/xmonad/xmonad/pull/268

.github/workflows/tests.yml

@@ -14,7 +14,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        resolver: [nightly]
+        resolver: [lts-16, lts-15, lts-14, lts-12]
 
     steps:
       - name: Clone project