Assertion when new-building base with tests enabled in cabal.project

When having a slightly patched Cabal (see #4209) to rebuild base, and `cabal.project` contains `tests: True`, then running `cabal new-build base` yields the following assertion [pkgStanzasEnabled `Set.isSubsetOf` pkgStanzasAvailable](https://github.com/haskell/cabal/blob/3d6dbe695c20c6bcceca77ebd8307ff0e3157762/cabal-install/Distribution/Client/ProjectPlanning.hs#L259) - though the sanity check code at head seems somewhat newer than the 1.24.0.2 code.

Adding

package base
tests: False

works around it.

Originally in [mail thread](https://mail.haskell.org/pipermail/haskell-cafe/2017-January/125895.html).

Reproduce `cabal new-build base` with a patched Cabal from #4209 and `tests: True` in `cabal.project`. Start at `cabal-install/Distribution/Client/ProjectPlanning.hs` near the `pkgStanzasEnabled Set.isSubsetOf pkgStanzasAvailable` assertion; done means the scenario no longer asserts while preserving the intended test setting.

Written by the indexing model from the issue text.