Distribution.Utils.TempTestDir is defined in Cabal but only used in testsuite

[mpickering]

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.

I noticed this when attempting to refactor withShorterPathForNewBuildStore function.