I noticed that Distribution.Utils.TempTestDir was only used in the
testsuite but defined in the Cabal library. Rather than expose this in
the public interface of the `Cabal` library, it is cleaner to refactor
it into a separate library (`Cabal-tests`) which can be used by any
testsuite component.

Also, in future it gives a clearer place to put utility functions which
need to be shared across the testsuite but not exposed in Cabal.
Cabal-tests can also freely add dependencies (such as exceptions) which
we might want to avoid adding to the Cabal library.

Fixes #9453