Add a fast version of the testsuite

The idea is to have a way to run as much of the testsuite as possible
in a short time, so that we'll run it more often (such as just before
checking in a change, for example).  'make fast' tries for good
coverage without using too many cycles.  Currently it takes about 4
minutes on a fast machine with an optimised GHC build; I think this
might still be a little on the slow side.

When you say 'make fast' in testsuite/tests/ghc-regress, we run each
test only one way, and all of the long-running tests are omitted.
Also, to get the runtime down further, I arbitrarily omitted many of
the should_run tests (because these tend to take a lot longer than
should_compile or should_fail tests).  I tried to keep a
representative few in each category.