Testsuite change: CLEANUP=1 is now the default

Hello devs, by popular request (#9758), the testsuite will now clean up after itself after running the tests, removing generated files, saving you some 10G of diskspace in the worst case. If you want the old behavior back, you can do: ``` $ make test CLEANUP=0 ``` I updated https://ghc.haskell.org/trac/ghc/wiki/Building/RunningTests/Running. The cleaning itself is not perfect at the moment; some temporary files are not deleted. I have a solution for this in https://ghc.haskell.org/trac/ghc/ticket/11980 and https://phabricator.haskell.org/D1187. More about that later. Cheers, Thomas